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Nixis across-platformpackage managerforUnix-likesystems, and a tool to instantiate and manage those systems, invented in 2003[6]by Eelco Dolstra. The Nix package manager employs a model in which software packages are each installed into unique directories with immutable contents. These directory names correspond tocryptographic hashesthat take into account all dependencies of a package, including other packages managed by Nix. As a result, Nix package names are content-identifying since packages with the same name will have had the same inputs and the same build platform, and therefore the same build result.[7] Package recipes for Nix are written in the purpose-built "Nix language", a declarative,purely functional,lazily evaluated,dynamically typedprogramming language.[8]Distinguishing features of the Nix language are strings with "context", string interpolation, first-class file system paths, and "indented strings", which in combination allow concisely expressing dependencies between file system data when specifying the contents of new files. Dependencies between files, as declared in the Nix language, are automatically tracked and persisted in the "Nix store".[9]New files in the Nix store are created through "derivations". A derivation is a persistent data structure that specifies an executable, arguments and environment variables for its invocation (seeexecve), and other files to be read from the Nix store. The executable is then run in a sandbox that prohibits access to anything but the explicitly specified input files and only allows writing to the designated output path. Nix preserves dependency information in output files by scanning for the distinctive hashes used for package directory names.[7] Automatic reference tracking ensures integrity of packages, even when they are transferred across machines. It also enablesgarbage collectionof unused packages when no other package depends on them. At the cost of greater storage requirements, all upgrades in Nix are guaranteed to be bothatomicand capable of efficientrollback. Unique directory names allow installing many packages with differing versions of shared libraries, and is claimed to eliminate so-calleddependency hell.[10]This also lets multiple users safely install software on the same system withoutadministrator privileges. As a result, the Nix package management and deployment model advertises more reliable,reproducible, and portable packages.[7][11] Nix has full support forLinux,macOS, andWSL, and can safely be installed side-by-side with another package manager. Nixpkgs is the package repository built upon the Nix package manager. According toRepology, as of January 2025[update]it contains more than 122,000 packages[12]and has a higher number of up-to-date packages than any other package repository.[13]Operating systems supported by Nixpkgs are primarily Linux and Darwin, with some support for Windows and BSD variants. Supported CPU architectures include64-bit x86andARM. Packages for these architectures are built regularly, using acontinuous integrationservice called Hydra,[14]and the results of these builds are uploaded to a public binary cache.[15]When Nix installs a package, it checks this cache and downloads the binary package to avoid building it locally. Nixpkgs is developed in a singleGitrepository on GitHub.[16]Beside packages, it also contains the source code forNixOS. NixOS[17]is aLinux distributionthat uses Nix for managing the entire system configuration, including the Linux kernel.[18] Nix is used for software packaging and distribution inCERN'sLHCb experiment.[19]Nix underlies the distributed software development platformsReplit[20]andFirebase Studio.[21] In 2021, a reimplementation by the nameTvixwas announced,[22]with the goals of modularity, full compatibility with Nixpkgs, and improved evaluator performance. As of 2024, Tvix has an evaluator[23]and a store implementation,[24]though the authors do not consider the project yet stable or ready for use in production.[25][non-primary source needed]Tvix is written primarily inRust.[26] In 2024, a team of volunteers released the first version ofLix,[27][non-primary source needed]a fork of Nix focused on correctness and compatibility that uses theMesonbuild automation system. The project intends to gradually rewrite parts of the code in Rust.[27]	https://en.wikipedia.org/wiki/Nix_package_manager
On March 22, 2016,software engineerAzer Koçulu took down theleft-padpackage that he had published tonpm(aJavaScriptpackage manager). Koçulu deleted the package after a dispute withKik Messenger, in which the company forcibly took control of the package namekik. As a result, thousands of software projects that usedleft-padas adependency, including theBabeltranscompilerand theReactweb framework, were unable to bebuiltor installed. This caused widespread disruption, as technology corporations small and large, includingFacebook,PayPal,NetflixandSpotify, usedleft-padin their software products. Several hours after the package was removed from npm, the company behind the platform,npm, Inc, manually restored the package. Later, npm disabled the ability to remove a package if more than 24 hours have elapsed since its publishing date and at least one other project depends on it. The incident drew widespread media attention and reactions from people in thesoftware industry. The removal ofleft-padhas prompted discussion regarding the intentional self-sabotage of software to promotesocial justiceand brought attention to the elevated possibility ofsupply chain attacksinmodular programming. left-padwas afree and open-sourceJavaScriptpackagepublished by Azer Koçulu, an independentsoftware engineerbased in Oakland, California.[1]The package repetitively prependscharactersto astringusing aloop.[1]left-padhas been characterized as being extremely simple, consisting of only 11lines of code(when empty lines are discounted) in the final version authored by Koçulu.[2][3] Koçulu publishedleft-padonnpm, the defaultpackage managerforNode.js, a JavaScriptruntime environment.[4][2]Despite its relative obscurity,left-padwas heavily used; the package was used as adependencyby thousands of other software projects and reached over 15 million downloads prior to its removal.[5][6]Some of the projects that requiredleft-padto function were critical to the JavaScript ecosystem at the time. This includedBabel, atranscompilerthat enables backwards-compatible JavaScript code,Webpack, a module bundling system, and bothReactandReact Native, which areframeworkswidely used for the development ofwebsitesandmobile apps, respectively.[7][8][1] In addition toleft-pad, Koçulu also ownedkikon npm, which was a tool that allowed developers to set up templates for their projects.[1]On March 11, 2016, Kik Interactive, a Canadian company owning theinstant messagingplatformKik Messenger, contacted Koçulu, requesting that he relinquish control of thekikpackage due to the company's ownership of the "Kik"trademark.[9]Part of the correspondence included the following message from Kik: We don’t mean to be a dick about [thekikpackage], but it’s a registered Trademark in most countries around the world and if you actually release an open source project called kik, our trademark lawyers are going to be banging on your door and taking down your accounts and stuff like that — and we’d have no choice but to do all that because you have to enforce trademarks or you lose them. Can we not come to some sort of a compromise to get you to change the name without involving lawyers? Is there something we could do for you in compensation to get you to change the name?[3] Koçulu responded shortly after, refusing to change the name of his project, saying: hahah, you’re actually being a dick. so, fuck you. don’t e-mail me back.[3] Koçulu also requested US$30,000 as compensation "for the hassle of giving up with my pet project for [sic] bunch of corporate dicks".[1]On March 18, 2016, Isaac Z. Schlueter, the chief executive officer of npm, Inc., wrote to both Kik Interactive and Koçulu, stating that the ownership of thekikpackage would be manually transferred to Kik Interactive.[1] After Koçulu expressed his disappointment with npm, Inc.'s decision and stated that he no longer wished to be part of the platform, Schlueter provided him with a command to delete all 273 modules that he had registered.[9]Koçulu executed the command on March 22, 2016, removing every package he had previously released.[1]left-padwas one of the packages that was "unpublished", rendering it no longer publicly accessible on npm.[5]Theleft-padsoftware project and contents remained available onGitHub.[9] Users attempting tobuildor install any JavaScript project that usedleft-padas a dependency (including dependents such as Babel or Webpack) received a404 errorthat caused the process to fail.[1]Notable software technology corporations used the package, includingMeta Platforms,PayPal,NetflixandSpotify.[8]Kik Interactive's developers themselves faced build problems as a result of the package's removal.[1] An hour after he deleted the packages, Koçulu published a post onMedium("I've Just Liberated My Modules"), explaining that he had unpublished his software projects from npm to protest corporate interests in free and open-source software.[1] Soon after the deletion, other software developers began to post a flood of complaints, reactions, and workarounds on the project'sGitissue tracking system.[7][1] Maintainers of open-source projects, including Babel, releasedhotfixesto remove the dependencies that Koçulu had unpublished.[7]Several of Koçulu's other package names were quickly taken over by newly published packages.[3]For example, another developer recreated theleft-padpackage—but released it as version 1.0.0. Since Koçulu published his as version 0.0.3, users continued to encounter problems.[3] Around two hours after the originalleft-padpackage was removed, npm manually "un-un-published" the original 0.0.3 version by restoring a backup.[1]Laurie Voss, chief technology officer of npm, wrote that the company "picked the needs of the many" despite internal disagreements about whether the action was "the right call".[10] npm changed its policy on the removal of published packages to prevent deletion if more than 24 hours have elapsed since its release date and at least one other project requires it as a dependency.[11]On behalf of npm, community manager Ashley Williams apologized for the disruption caused by the incident, stating that the platform "[failed] to protect the community".[11]Kik Interactive also apologized for the incident, with the company's head of messaging Mike Roberts publishing the email chain with Koçulu on Medium and characterizing his interaction as a "polite request".[8]Roberts wrote that they had initially reached out to Koçulu because they wished to publish an open-source package on npm with the name Koçulu was using.[5]Koçulu stated that he was sorry for disrupting others' work, but he believed he did it "for the benefit of the community in [sic] long term".[2] The incident drew varied reactions from users onTwitter,GitHub,RedditandHacker News, with many claiming that it briefly "broke the Internet".[2][8][9][1]Many commented on the "move fast and break things" culture of JavaScript development, the unpredictable nature of open-source software, and a perceived over-reliance onmodular programming.[2][8][3]Users also expressed disappointment regarding npm's decision to forcefully transfer Koçulu's package to Kik Interactive over a legal threat.[1] The incident showed how the disruption of an npm package could lead to asupply chain attack. In addition to the widely publicizedleft-padincident, a number of individuals had immediately hijacked Koçulu's other packages with unknown code after they were removed.[7]npm released a new policy to prevent malicious takeovers in similar disputes,[3]but theleft-padincident is still cited as an example of over-reliance on external contributors leading to an increasedattack surfacefor software products.[12]Koçulu's intentional self-sabotage ofleft-padto highlight a social issue has also been described as a precursor to incidences ofprotestwarebeing published on platforms like npm.[6]	https://en.wikipedia.org/wiki/Npm_left-pad_incident
Incomputing, anabstraction layerorabstraction levelis a way of hiding the working details of a subsystem. Examples of software models that use layers of abstraction include theOSI modelfornetwork protocols,OpenGL, and othergraphics libraries, which allow theseparation of concernsto facilitateinteroperabilityandplatform independence. Incomputer science, an abstraction layer is a generalization of aconceptual modeloralgorithm, away from any specific implementation. These generalizations arise from broad similarities that are best encapsulated by models that express similarities present in various specific implementations. The simplification provided by a good abstraction layer allows for easy reuse by distilling a useful concept ordesign patternso that situations, where it may be accurately applied, can be quickly recognized. Just composing lower-level elements into a construct doesn't count as an abstraction layer unless it shields users from its underlying complexity.[1] A layer is considered to be on top of another if itdependson it. Every layer can exist without the layers above it, and requires the layers below it to function. Frequently abstraction layers can be composed into a hierarchy of abstraction levels. The OSI model comprises seven abstraction layers. Each layer of the model encapsulates and addresses a different part of the needs of digital communications, thereby reducing the complexity of the associated engineering solutions. Afamous aphorismofDavid Wheeleris, "All problems in computer science can be solved by another level ofindirection."[2]This is often deliberately misquoted with "abstraction" substituted for "indirection."[citation needed]It is also sometimes misattributed toButler Lampson.Kevlin Henney's corollary to this is, "...except for the problem of too many layers of indirection."[3] In acomputer architecture, a computer system is usually represented as consisting of several abstraction levels such as: Programmable logic is often considered part of the hardware, while the logical definitions are also sometimes seen as part of a device's software or firmware. Firmware may include only low-level software, but can also include all software, including an operating system and applications. The software layers can be further divided into hardware abstraction layers, physical and logical device drivers, repositories such as filesystems, operating system kernels, middleware, applications, and others. A distinction can also be made from low-level programming languages likeVHDL,machine language,assembly languageto acompiled language,interpreter, andscript language.[4] In the Unix operating system, most types of input and output operations are considered to be streams of bytes read from a device or written to a device. This stream of bytes model is used for file I/O, socket I/O, and terminal I/O in order to provide device independence. In order to read and write to a device at the application level, the program calls a function to open the device, which may be a real device such as a terminal or a virtual device such as a network port or a file in a file system. The device's physical characteristics are mediated by the operating system which in turn presents an abstract interface that allows the programmer to read and write bytes from/to the device. The operating system then performs the actual transformation needed to read and write the stream of bytes to the device. Most graphics libraries such as OpenGL provide an abstract graphical device model as an interface. The library is responsible for translating the commands provided by the programmer into the specific device commands needed to draw the graphical elements and objects. The specific device commands for aplotterare different from the device commands for aCRT monitor, but the graphics library hides the implementation and device-dependent details by providing an abstract interface which provides a set ofprimitivesthat are generally useful for drawing graphical objects.	https://en.wikipedia.org/wiki/Abstraction_layer
Thearchetype patternis asoftware design patternthat separates logic from implementation. The separation is accomplished through the creation of twoabstract classes: adecorator(for logic), and a delegate (for implementation). TheFactoryhandles the mapping of decorator and delegate classes and returns the pair associated with aparameteror parameters passed. Theinterfaceis the contract between a decorator, a delegate, and the calling class, creating anInversion of Responsibility.[1]This example use two branches; however, you can have 'N' branches as required. The pattern means that one branch from the interface does not have to worry about how another branch operators as long it implements the interface. DecoratorThe descendants of the decorator class handle the logic, for example performing a calculation. The descendants of the decorator can then call the descendants of the delegated when or if they wish to pass responsibility for example storage or communication. DelegateThe descendants of the delegate flow class handle the implementation for call a sub-system, storage, or communication. Different children can use completely different sub-systems storage, or communications than each other. Delegation pattern- calls the specific implementation Decorator pattern- performs the generalised logic Factory method pattern- creates the archetype combination	https://en.wikipedia.org/wiki/Archetype_pattern
Incomputing,aspect-oriented programming(AOP) is aprogramming paradigmthat aims to increasemodularityby allowing theseparation ofcross-cutting concerns. It does so by adding behavior to existing code (anadvice)withoutmodifying the code, instead separately specifying which code is modified via a "pointcut" specification, such as "log all function calls when the function's name begins with 'set'". This allows behaviors that are not central to thebusiness logic(such as logging) to be added to a program without cluttering the code of core functions. AOP includes programming methods and tools that support the modularization of concerns at the level of the source code, whileaspect-oriented software developmentrefers to a whole engineering discipline. Aspect-oriented programming entails breaking down program logic into cohesive areas of functionality (so-calledconcerns). Nearly all programming paradigms support some level of grouping andencapsulationof concerns into separate, independent entities by providing abstractions (e.g., functions, procedures, modules, classes, methods) that can be used for implementing, abstracting, and composing these concerns. Some concerns "cut across" multiple abstractions in a program, and defy these forms of implementation. These concerns are calledcross-cutting concernsor horizontal concerns. Loggingexemplifies a cross-cutting concern because a logging strategy must affect every logged part of the system. Logging therebycrosscutsall logged classes and methods. All AOP implementations have some cross-cutting expressions that encapsulate each concern in one place. The difference between implementations lies in the power, safety, and usability of the constructs provided. For example, interceptors that specify the methods to express a limited form of cross-cutting, without much support for type-safety or debugging.AspectJhas a number of such expressions and encapsulates them in a special class, called anaspect. For example, an aspect can alter the behavior of the base code (the non-aspect part of a program) by applyingadvice(additional behavior) at variousjoin points(points in a program) specified in a quantification or query called apointcut(that detects whether a given join point matches). An aspect can also make binary-compatible structural changes to other classes, such as adding members or parents. AOP has several direct antecedents A1 and A2:[1]reflectionandmetaobjectprotocols,subject-oriented programming, Composition Filters, and Adaptive Programming.[2] Gregor Kiczalesand colleagues atXerox PARCdeveloped the explicit concept of AOP and followed this with theAspectJAOP extension to Java. IBM's research team pursued a tool approach over a language design approach and in 2001 proposedHyper/Jand theConcern Manipulation Environment, which have not seen wide use. The examples in this article use AspectJ. TheMicrosoft Transaction Serveris considered to be the first major application of AOP followed byEnterprise JavaBeans.[3][4] Typically, an aspect isscatteredortangledas code, making it harder to understand and maintain. It is scattered by the function (such as logging) being spread over a number of unrelated functions that might useitsfunction, possibly in entirely unrelated systems or written in different languages. Thus, changing logging can require modifying all affected modules. Aspects become tangled not only with the mainline function of the systems in which they are expressed but also with each other. Changing one concern thus entails understanding all the tangled concerns or having some means by which the effect of changes can be inferred. For example, consider a banking application with a conceptually very simple method for transferring an amount from one account to another:[5] However, this transfer method overlooks certain considerations that a deployed application would require, such as verifying that the current user is authorized to perform this operation, encapsulatingdatabase transactionsto prevent accidental data loss, and logging the operation for diagnostic purposes. A version with all those new concerns might look like this: In this example, other interests have becometangledwith the basic functionality (sometimes called thebusiness logic concern). Transactions, security, and logging all exemplifycross-cutting concerns. Now consider what would happen if we suddenly need to change the security considerations for the application. In the program's current version, security-related operations appearscatteredacross numerous methods, and such a change would require major effort. AOP tries to solve this problem by allowing the programmer to express cross-cutting concerns in stand-alone modules calledaspects. Aspects can containadvice(code joined to specified points in the program) andinter-type declarations(structural members added to other classes). For example, a security module can include advice that performs a security check before accessing a bank account. Thepointcutdefines the times (join points) when one can access a bank account, and the code in the advice body defines how the security check is implemented. That way, both the check and the places can be maintained in one place. Further, a good pointcut can anticipate later program changes, so if another developer creates a new method to access the bank account, the advice will apply to the new method when it executes. So for the example above implementing logging in an aspect: One can think of AOP as a debugging tool or a user-level tool. Advice should be reserved for cases in which one cannot get the function changed (user level)[6]or do not want to change the function in production code (debugging). The advice-related component of an aspect-oriented language defines a join point model (JPM). A JPM defines three things: Join-point models can be compared based on the join points exposed, how join points are specified, the operations permitted at the join points, and the structural enhancements that can be expressed. "Kinded" PCDs match a particular kind of join point (e.g., method execution) and often take a Java-like signature as input. One such pointcut looks like this: This pointcut matches a method-execution join point, if the method name starts with "set" and there is exactly one argument of any type. "Dynamic" PCDs check runtime types and bind variables. For example, This pointcut matches when the currently executing object is an instance of classPoint. Note that the unqualified name of a class can be used via Java's normal type lookup. "Scope" PCDs limit the lexical scope of the join point. For example: This pointcut matches any join point in any type in thecom.companypackage. The*is one form of the wildcards that can be used to match many things with one signature. Pointcuts can be composed and named for reuse. For example: There are other kinds of JPMs. All advice languages can be defined in terms of their JPM. For example, a hypothetical aspect language forUMLmay have the following JPM: Inter-type declarationsprovide a way to express cross-cutting concerns affecting the structure of modules. Also known asopen classesandextension methods, this enables programmers to declare in one place members or parents of another class, typically to combine all the code related to a concern in one aspect. For example, if a programmer implemented the cross-cutting display-update concern using visitors, an inter-type declaration using thevisitor patternmight look like this in AspectJ: This code snippet adds theacceptVisitormethod to thePointclass. Any structural additions are required to be compatible with the original class, so that clients of the existing class continue to operate, unless the AOP implementation can expect to control all clients at all times. AOP programs can affect other programs in two different ways, depending on the underlying languages and environments: The difficulty of changing environments means most implementations produce compatible combination programs through a type ofprogram transformationknown asweaving. Anaspect weaverreads the aspect-oriented code and generates appropriate object-oriented code with the aspects integrated. The same AOP language can be implemented through a variety of weaving methods, so the semantics of a language should never be understood in terms of the weaving implementation. Only the speed of an implementation and its ease of deployment are affected by the method of combination used. Systems can implement source-level weaving using preprocessors (as C++ was implemented originally inCFront) that require access to program source files. However, Java's well-defined binary form enables bytecode weavers to work with any Java program in .class-file form. Bytecode weavers can be deployed during the build process or, if the weave model is per-class, during class loading.AspectJstarted with source-level weaving in 2001, delivered a per-class bytecode weaver in 2002, and offered advanced load-time support after the integration ofAspectWerkzin 2005. Any solution that combines programs at runtime must provide views that segregate them properly to maintain the programmer's segregated model. Java's bytecode support for multiple source files enables any debugger to step through a properly woven .class file in a source editor. However, some third-party decompilers cannot process woven code because they expect code produced by Javac rather than all supported bytecode forms (see also§ Criticism, below). Deploy-timeweaving offers another approach.[7]This basically implies post-processing, but rather than patching the generated code, this weaving approachsubclassesexisting classes so that the modifications are introduced by method-overriding. The existing classes remain untouched, even at runtime, and all existing tools, such as debuggers and profilers, can be used during development. A similar approach has already proven itself in the implementation of manyJava EEapplication servers, such asIBM'sWebSphere. Standard terminology used in Aspect-oriented programming may include: Aspects emerged fromobject-oriented programmingandreflective programming. AOP languages have functionality similar to, but more restricted than,metaobject protocols. Aspects relate closely to programming concepts likesubjects,mixins, anddelegation. Other ways to use aspect-oriented programming paradigms includeComposition Filtersand thehyperslicesapproach. Since at least the 1970s, developers have been using forms of interception and dispatch-patching that resemble some of the implementation methods for AOP, but these never had the semantics that the cross-cutting specifications provide in one place.[citation needed] Designers have considered alternative ways to achieve separation of code, such asC#'s partial types, but such approaches lack a quantification mechanism that allows reaching several join points of the code with one declarative statement.[citation needed] Though it may seem unrelated, in testing, the use of mocks or stubs requires the use of AOP techniques, such as around advice. Here the collaborating objects are for the purpose of the test, a cross-cutting concern. Thus, the various Mock Object frameworks provide these features. For example, a process invokes a service to get a balance amount. In the test of the process, it is unimportant where the amount comes from, but only that the process uses the balance according to the requirements.[citation needed] Programmers need to be able to read and understand code to prevent errors.[10]Even with proper education, understanding cross-cutting concerns can be difficult without proper support for visualizing both static structure and the dynamic flow of a program.[11]Starting in 2002, AspectJ began to provide IDE plug-ins to support the visualizing of cross-cutting concerns. Those features, as well as aspect code assist andrefactoring, are now common. Given the power of AOP, making a logical mistake in expressing cross-cutting can lead to widespread program failure. Conversely, another programmer may change the join points in a program, such as by renaming or moving methods, in ways that the aspect writer did not anticipate and withunforeseen consequences. One advantage of modularizing cross-cutting concerns is enabling one programmer to easily affect the entire system. As a result, such problems manifest as a conflict over responsibility between two or more developers for a given failure. AOP can expedite solving these problems, as only the aspect must be changed. Without AOP, the corresponding problems can be much more spread out.[citation needed] The most basic criticism of the effect of AOP is that control flow is obscured, and that it is not only worse than the much-malignedGOTOstatement, but is closely analogous to the jokeCOME FROMstatement.[11]Theobliviousness of application, which is fundamental to many definitions of AOP (the code in question has no indication that an advice will be applied, which is specified instead in the pointcut), means that the advice is not visible, in contrast to an explicit method call.[11][12]For example, compare the COME FROM program:[11] with an AOP fragment with analogous semantics: Indeed, the pointcut may depend on runtime condition and thus not be statically deterministic. This can be mitigated but not solved by static analysis and IDE support showing which advicespotentiallymatch. General criticisms are that AOP purports to improve "both modularity and the structure of code", but some counter that it instead undermines these goals and impedes "independent development and understandability of programs".[13]Specifically, quantification by pointcuts breaks modularity: "one must, in general, have whole-program knowledge to reason about the dynamic execution of an aspect-oriented program."[14]Further, while its goals (modularizing cross-cutting concerns) are well understood, its actual definition is unclear and not clearly distinguished from other well-established techniques.[13]Cross-cutting concerns potentially cross-cut each other, requiring some resolution mechanism, such as ordering.[13]Indeed, aspects can apply to themselves, leading to problems such as theliar paradox.[15] Technical criticisms include that the quantification of pointcuts (defining where advices are executed) is "extremely sensitive to changes in the program", which is known as thefragile pointcut problem.[13]The problems with pointcuts are deemed intractable. If one replaces the quantification of pointcuts with explicit annotations, one obtainsattribute-oriented programminginstead, which is simply an explicit subroutine call and suffers the identical problem of scattering, which AOP was designed to solve.[13] Manyprogramming languageshave implemented AOP, within the language, or as an externallibrary, including:	https://en.wikipedia.org/wiki/Aspect-oriented_programming
Incomputer programming, acallbackis afunctionthat is stored as data (areference) and designed to be called by another function – oftenbackto the originalabstraction layer. A function that accepts a callbackparametermay be designed to call back beforereturningto its caller which is known assynchronousorblocking. The function that accepts a callback may be designed to store the callback so that it can be called back after returning which is known asasynchronous,non-blockingordeferred. Programming languagessupport callbacks in different ways such asfunction pointers,lambda expressionsandblocks. A callback can be likened to leaving instructions with a tailor for what to do when a suit is ready, such as calling a specific phone number or delivering it to a given address. These instructions represent a callback: a function provided in advance to be executed later, often by a different part of the system and not necessarily by the one that received it. The termcallbackcan be misleading, as it does not necessarily imply a return to the original caller, unlike atelephone callback. A blocking callback runs in theexecutioncontext of the function that passes the callback. A deferred callback can run in a different context such as duringinterruptor from athread. As such, a deferred callback can be used for synchronization and delegating work to another thread. A callback can be used for event handling. Often, consuming code registers a callback for a particular type of event. When that event occurs, the callback is called. Callbacks are often used to program thegraphical user interface(GUI) of a program that runs in awindowing system. The application supplies a reference to a custom callback function for the windowing system to call. The windowing system calls this function to notify the application of events likemouseclicks andkeypresses. A callback can be used to implement asynchronous processing. A caller requests an action and provides a callback to be called when the action completes which might be long after the request is made. A callback can be used to implementpolymorphism. In the following pseudocode,say_hican take eitherwrite_statusorwrite_error. The callback technology is implemented differently byprogramming language. Inassembly,C,C++,Pascal,Modula2and other languages, a callback function is stored internally as afunction pointer. Using the same storage allows different languages to directly share callbacks without adesign-time or runtimeinteroperabilitylayer. For example, theWindows APIis accessible via multiple languages, compilers and assemblers. C++ also allows objects to provide an implementation of the function call operation. TheStandard Template Libraryaccepts these objects (calledfunctors) as parameters. Manydynamic languages, such asJavaScript,Lua,Python,Perl[1][2]andPHP, allow a function object to be passed. CLI languagessuch asC#andVB.NETprovide atype-safeencapsulating function reference known asdelegate. Events andevent handlers, as used in .NET languages, provide for callbacks. Functional languages generally supportfirst-class functions, which can be passed as callbacks to other functions, stored as data or returned from functions. Many languages, including Perl, Python,Ruby,Smalltalk,C++(11+), C# and VB.NET (new versions) and most functional languages, supportlambda expressions, unnamed functions with inline syntax, that generally acts as callbacks.. In some languages, includingScheme,ML, JavaScript, Perl, Python, Smalltalk, PHP (since 5.3.0),[3]C++ (11+), Java (since 8),[4]and many others, a lambda can be aclosure, i.e. can access variables locally defined in the context in which the lambda is defined. In anobject-oriented programminglanguage such asJavaversions before function-valued arguments, the behavior of a callback can be achieved by passing an object that implements an interface. The methods of this object are callbacks. InPL/IandALGOL 60a callback procedure may need to be able to access local variables in containing blocks, so it is called through anentry variablecontaining both the entry point and context information.[5] Callbacks have a wide variety of uses, for example in error signaling: aUnixprogram might not want to terminate immediately when it receivesSIGTERM, so to make sure that its termination is handled properly, it would register the cleanup function as a callback. Callbacks may also be used to control whether a function acts or not:Xliballows custom predicates to be specified to determine whether a program wishes to handle an event. In the followingCcode, functionprint_numberuses parameterget_numberas a blocking callback.print_numberis called withget_answer_to_most_important_questionwhich acts as a callback function. When run the output is: "Value: 42". In C++,functorcan be used in addition to function pointer. In the followingC#code, methodHelper.Methoduses parametercallbackas a blocking callback.Helper.Methodis called withLogwhich acts as a callback function. When run, the following is written to the console: "Callback was: Hello world". In the followingKotlincode, functionaskAndAnsweruses parametergetAnsweras a blocking callback.askAndAnsweris called withgetAnswerToMostImportantQuestionwhich acts as a callback function. Running this will tell the user that the answer to their question is "42". In the followingJavaScriptcode, functioncalculateuses parameteroperateas a blocking callback.calculateis called withmultiplyand then withsumwhich act as callback functions. The collection method.each()of thejQuerylibraryuses the function passed to it as a blocking callback. It calls the callback for each item of the collection. For example: Deferred callbacks are commonly used for handling events from the user, the client and timers. Examples can be found inaddEventListener,AjaxandXMLHttpRequest.[6] In addition to using callbacks in JavaScript source code, C functions that take a function are supported via js-ctypes.[7] The followingREBOL/Redcode demonstrates callback use. Rusthave theFn,FnMutandFnOncetraits.[8] In thisLuacode, functioncalculateaccepts theoperationparameter which is used as a blocking callback.calculateis called with bothaddandmultiply, and then uses ananonymous functionto divide. In the followingPythoncode, functioncalculateaccepts a parameteroperatethat is used as a blocking callback.calculateis called withsquarewhich acts as a callback function. In the followingJuliacode, functioncalculateaccepts a parameteroperatethat is used as a blocking callback.calculateis called withsquarewhich acts as a callback function.	https://en.wikipedia.org/wiki/Callback_(computer_science)
Inprogramming languages, aclosure, alsolexical closureorfunction closure, is a technique for implementinglexically scopedname bindingin a language withfirst-class functions.Operationally, a closure is arecordstoring afunction[a]together with an environment.[1]The environment is a mapping associating eachfree variableof the function (variables that are used locally, but defined in an enclosing scope) with thevalueorreferenceto which the name was bound when the closure was created.[b]Unlike a plain function, a closure allows the function to access thosecaptured variablesthrough the closure's copies of their values or references, even when the function is invoked outside their scope. The concept of closures was developed in the 1960s for the mechanical evaluation of expressions in theλ-calculusand was first fully implemented in 1970 as a language feature in thePAL programming languageto support lexically scopedfirst-class functions.[2] Peter Landindefined the termclosurein 1964 as having anenvironment partand acontrol partas used by hisSECD machinefor evaluating expressions.[3]Joel Mosescredits Landin with introducing the termclosureto refer to alambda expressionwith open bindings (free variables) that have been closed by (or bound in) the lexical environment, resulting in aclosed expression, or closure.[4][5]This use was subsequently adopted bySussmanandSteelewhen they definedSchemein 1975,[6]a lexically scoped variant ofLisp, and became widespread. Sussman andAbelsonalso use the termclosurein the 1980s with a second, unrelated meaning: the property of an operator that adds data to adata structureto also be able to add nested data structures. This use of the term comes frommathematics use, rather than the prior use in computer science. The authors consider this overlap in terminology to be "unfortunate."[7] The termclosureis often used as a synonym foranonymous function, though strictly, an anonymous function is a functionliteralwithout a name, while a closure is an instance of a function, avalue, whose non-local variables have been bound either to values or tostorage locations(depending on the language; see thelexical environmentsection below). For example, in the followingPythoncode: the values ofaandbare closures, in both cases produced by returning anested functionwith a free variable from the enclosing function, so that the free variable binds to the value of parameterxof the enclosing function. The closures inaandbare functionally identical. The only difference in implementation is that in the first case we used a nested function with a name,g, while in the second case we used an anonymous nested function (using the Python keywordlambdafor creating an anonymous function). The original name, if any, used in defining them is irrelevant. A closure is a value like any other value. It does not need to be assigned to a variable and can instead be used directly, as shown in the last two lines of the example. This usage may be deemed an "anonymous closure". The nested function definitions are not themselves closures: they have a free variable which is not yet bound. Only once the enclosing function is evaluated with a value for the parameter is the free variable of the nested function bound, creating a closure, which is then returned from the enclosing function. Lastly, a closure is only distinct from a function with free variables when outside of the scope of the non-local variables, otherwise the defining environment and the execution environment coincide and there is nothing to distinguish these (static and dynamic binding cannot be distinguished because the names resolve to the same values). For example, in the program below, functions with a free variablex(bound to the non-local variablexwith global scope) are executed in the same environment wherexis defined, so it is immaterial whether these are actually closures: This is most often achieved by a function return, since the function must be defined within the scope of the non-local variables, in which case typically its own scope will be smaller. This can also be achieved byvariable shadowing(which reduces the scope of the non-local variable), though this is less common in practice, as it is less useful and shadowing is discouraged. In this examplefcan be seen to be a closure becausexin the body offis bound to thexin the global namespace, not thexlocal tog: The use of closures is associated with languages where functions arefirst-class objects, in which functions can be returned as results fromhigher-order functions, or passed as arguments to other function calls; if functions with free variables are first-class, then returning one creates a closure. This includesfunctional programminglanguages such asLispandML, and many modern, multi-paradigm languages, such asJulia,Python, andRust. Closures are also often used withcallbacks, particularly forevent handlers, such as inJavaScript, where they are used for interactions with adynamic web page. Closures can also be used in acontinuation-passing styletohide state. Constructs such asobjectsandcontrol structurescan thus be implemented with closures. In some languages, a closure may occur when a function is defined within another function, and the inner function refers to local variables of the outer function. Atrun-time, when the outer function executes, a closure is formed, consisting of the inner function's code and references (the upvalues) to any variables of the outer function required by the closure. Closures typically appear in languages withfirst-class functions—in other words, such languages enable functions to be passed as arguments, returned from function calls, bound to variable names, etc., just like simpler types such as strings and integers. For example, consider the followingSchemefunction: In this example, thelambda expression(lambda (book) (>= (book-sales book) threshold))appears within the functionbest-selling-books. When the lambda expression is evaluated, Scheme creates a closure consisting of the code for the lambda expression and a reference to thethresholdvariable, which is afree variableinside the lambda expression. The closure is then passed to thefilterfunction, which calls it repeatedly to determine which books are to be added to the result list and which are to be discarded. Because the closure has a reference tothreshold, it can use that variable each timefiltercalls it. The functionfiltermight be defined in a separate file. Here is the same example rewritten inJavaScript, another popular language with support for closures: The arrow operator=>is used to define anarrow function expression, and anArray.filtermethod[8]instead of a globalfilterfunction, but otherwise the structure and the effect of the code are the same. A function may create a closure and return it, as in this example: Because the closure in this case outlives the execution of the function that creates it, the variablesfanddxlive on after the functionderivativereturns, even though execution has left their scope and they are no longer visible. In languages without closures, the lifetime of an automatic local variable coincides with the execution of the stack frame where that variable is declared. In languages with closures, variables must continue to exist as long as any existing closures have references to them. This is most commonly implemented using some form ofgarbage collection. A closure can be used to associate a function with a set of "private" variables, which persist over several invocations of the function. Thescopeof the variable encompasses only the closed-over function, so it cannot be accessed from other program code. These are analogous toprivate variablesinobject-oriented programming, and in fact closures are similar to statefulfunction objects(or functors) with a single call-operator method. In stateful languages, closures can thus be used to implement paradigms for state representation andinformation hiding, since the closure's upvalues (its closed-over variables) are of indefiniteextent, so a value established in one invocation remains available in the next. Closures used in this way no longer havereferential transparency, and are thus no longerpure functions; nevertheless, they are commonly used in impure functional languages such asScheme. Closures have many uses: Note: Some speakers call any data structure that binds alexicalenvironment a closure, but the term usually refers specifically to functions. Closures are typically implemented with a specialdata structurethat contains apointer to the function code, plus a representation of the function's lexical environment (i.e., the set of available variables) at the time when the closure was created. The referencing environmentbindsthe non-local names to the corresponding variables in the lexical environment at the time the closure is created, additionally extending their lifetime to at least as long as the lifetime of the closure. When the closure is entered at a later time, possibly with a different lexical environment, the function is executed with its non-local variables referring to the ones captured by the closure, not the current environment. A language implementation cannot easily support full closures if its run-time memory model allocates allautomatic variableson a linearstack. In such languages, a function's automatic local variables are deallocated when the function returns. However, a closure requires that the free variables it references survive the enclosing function's execution. Therefore, those variables must be allocated so that they persist until no longer needed, typically viaheap allocation, rather than on the stack, and their lifetime must be managed so they survive until all closures referencing them are no longer in use. This explains why, typically, languages that natively support closures also usegarbage collection. The alternatives are manual memory management of non-local variables (explicitly allocating on the heap and freeing when done), or, if using stack allocation, for the language to accept that certain use cases will lead toundefined behaviour, due todangling pointersto freed automatic variables, as in lambda expressions in C++11[10]or nested functions in GNU C.[11]Thefunarg problem(or "functional argument" problem) describes the difficulty of implementing functions as first class objects in a stack-based programming language such as C or C++. Similarly inDversion 1, it is assumed that the programmer knows what to do withdelegatesand automatic local variables, as their references will be invalid after return from its definition scope (automatic local variables are on the stack) – this still permits many useful functional patterns, but for complex cases needs explicitheap allocationfor variables. D version 2 solved this by detecting which variables must be stored on the heap, and performs automatic allocation. Because D uses garbage collection, in both versions, there is no need to track usage of variables as they are passed. In strict functional languages with immutable data (e.g.Erlang), it is very easy to implement automatic memory management (garbage collection), as there are no possible cycles in variables' references. For example, in Erlang, all arguments and variables are allocated on the heap, but references to them are additionally stored on the stack. After a function returns, references are still valid. Heap cleaning is done by incremental garbage collector. In ML, local variables are lexically scoped, and hence define a stack-like model, but since they are bound to values and not to objects, an implementation is free to copy these values into the closure's data structure in a way that is invisible to the programmer. Scheme, which has anALGOL-like lexical scope system with dynamic variables and garbage collection, lacks a stack programming model and does not suffer from the limitations of stack-based languages. Closures are expressed naturally in Scheme. The lambda form encloses the code, and the free variables of its environment persist within the program as long as they can possibly be accessed, and so they can be used as freely as any other Scheme expression.[citation needed] Closures are closely related to Actors in theActor modelof concurrent computation where the values in the function's lexical environment are calledacquaintances. An important issue for closures inconcurrent programminglanguages is whether the variables in a closure can be updated and, if so, how these updates can be synchronized. Actors provide one solution.[12] Closures are closely related tofunction objects; the transformation from the former to the latter is known asdefunctionalizationorlambda lifting; see alsoclosure conversion.[citation needed] As different languages do not always have a common definition of the lexical environment, their definitions of closure may vary also. The commonly held minimalist definition of the lexical environment defines it as a set of allbindings of variablesin the scope, and that is also what closures in any language have to capture. However the meaning of avariablebinding also differs. In imperative languages, variables bind to relative locations in memory that can store values. Although the relative location of a binding does not change at runtime, the value in the bound location can. In such languages, since closure captures the binding, any operation on the variable, whether done from the closure or not, are performed on the same relative memory location. This is often called capturing the variable "by reference". Here is an example illustrating the concept inECMAScript, which is one such language: Functionfooand the closures referred to by variablesfandgall use the same relative memory location signified by local variablex. In some instances the above behaviour may be undesirable, and it is necessary to bind a different lexical closure. Again in ECMAScript, this would be done using theFunction.bind(). [13] For this example the expected behaviour would be that each link should emit its id when clicked; but because the variable 'e' is bound to the scope above, and lazy evaluated on click, what actually happens is that each on click event emits the id of the last element in 'elements' bound at the end of the for loop.[14] Again here variableewould need to be bound by the scope of the block usinghandle.bind(this)or theletkeyword. On the other hand, many functional languages, such asML, bind variables directly to values. In this case, since there is no way to change the value of the variable once it is bound, there is no need to share the state between closures—they just use the same values. This is often called capturing the variable "by value". Java's local and anonymous classes also fall into this category—they require captured local variables to befinal, which also means there is no need to share state. Some languages enable choosing between capturing the value of a variable or its location. For example, in C++11, captured variables are either declared with[&], which means captured by reference, or with[=], which means captured by value. Yet another subset,lazyfunctional languages such asHaskell, bind variables to results of future computations rather than values. Consider this example in Haskell: The binding ofrcaptured by the closure defined within functionfoois to the computation(x / y)—which in this case results in division by zero. However, since it is the computation that is captured, and not the value, the error only manifests when the closure is invoked, and then attempts to use the captured binding. Yet more differences manifest themselves in the behavior of other lexically scoped constructs, such asreturn,breakandcontinuestatements. Such constructs can, in general, be considered in terms of invoking anescape continuationestablished by an enclosing control statement (in case ofbreakandcontinue, such interpretation requires looping constructs to be considered in terms of recursive function calls). In some languages, such as ECMAScript,returnrefers to the continuation established by the closure lexically innermost with respect to the statement—thus, areturnwithin a closure transfers control to the code that called it. However, inSmalltalk, the superficially similar operator^invokes the escape continuation established for the method invocation, ignoring the escape continuations of any intervening nested closures. The escape continuation of a particular closure can only be invoked in Smalltalk implicitly by reaching the end of the closure's code. These examples in ECMAScript and Smalltalk highlight the difference: The above code snippets will behave differently because the Smalltalk^operator and the JavaScriptreturnoperator are not analogous. In the ECMAScript example,return xwill leave the inner closure to begin a new iteration of theforEachloop, whereas in the Smalltalk example,^xwill abort the loop and return from the methodfoo. Common Lispprovides a construct that can express either of the above actions: Lisp(return-from foo x)behaves asSmalltalk^x, while Lisp(return-from nil x)behaves asJavaScriptreturn x. Hence, Smalltalk makes it possible for a captured escape continuation to outlive the extent in which it can be successfully invoked. Consider: When the closure returned by the methodfoois invoked, it attempts to return a value from the invocation offoothat created the closure. Since that call has already returned and the Smalltalk method invocation model does not follow thespaghetti stackdiscipline to facilitate multiple returns, this operation results in an error. Some languages, such asRuby, enable the programmer to choose the wayreturnis captured. An example in Ruby: BothProc.newandlambdain this example are ways to create a closure, but semantics of the closures thus created are different with respect to thereturnstatement. InScheme, definition and scope of thereturncontrol statement is explicit (and only arbitrarily named 'return' for the sake of the example). The following is a direct translation of the Ruby sample. Some languages have features which simulate the behavior of closures. In languages such asC++,C#,D,Java,Objective-C, andVisual Basic (.NET)(VB.NET), these features are the result of the language's object-oriented paradigm. SomeClibraries supportcallbacks. This is sometimes implemented by providing two values when registering the callback with the library: a function pointer and a separatevoidpointer to arbitrary data of the user's choice. When the library executes the callback function, it passes along the data pointer. This enables the callback to maintain state and to refer to information captured at the time it was registered with the library. The idiom is similar to closures in functionality, but not in syntax. Thevoidpointer is nottype safeso this C idiom differs from type-safe closures in C#, Haskell or ML. Callbacks are used extensively ingraphical user interface(GUI)widget toolkitsto implementevent-driven programmingby associating general functions of graphical widgets (menus, buttons, check boxes, sliders, spinners, etc.) with application-specific functions implementing the specific desired behavior for the application. With aGNU Compiler Collection(GCC) extension, a nested function[15]can be used and a function pointer can emulate closures, provided the function does not exit the containing scope. The next example is invalid becauseadderis a top-level definition (depending on compiler version, it could produce a correct result if compiled with no optimizing, i.e., at-O0): But movingadder(and, optionally, thetypedef) inmainmakes it valid: If executed this now prints11as expected. Javaenablesclassesto be defined insidemethods. These are calledlocal classes. When such classes are not named, they are known asanonymous classes(or anonymousinnerclasses). A local class (either named or anonymous) may refer to names in lexically enclosing classes, or read-only variables (marked asfinal) in the lexically enclosing method. The capturing offinalvariables enables capturing variables by value. Even if the variable to capture is non-final, it can always be copied to a temporaryfinalvariable just before the class. Capturing of variables by reference can be emulated by using afinalreference to a mutable container, for example, a one-element array. The local class will not be able to change the value of the container reference, but it will be able to change the contents of the container. With the advent of Java 8's lambda expressions,[16]the closure causes the above code to be executed as: Local classes are one of the types ofinner classthat are declared within the body of a method. Java also supports inner classes that are declared asnon-static membersof an enclosing class.[17]They are normally referred to just as "inner classes".[18]These are defined in the body of the enclosing class and have full access to instance variables of the enclosing class. Due to their binding to these instance variables, an inner class may only be instantiated with an explicit binding to an instance of the enclosing class using a special syntax.[19] Upon execution, this will print the integers from 0 to 9. Beware to not confuse this type of class with the nested class, which is declared in the same way with an accompanied usage of the "static" modifier; those have not the desired effect but are instead just classes with no special binding defined in an enclosing class. As ofJava 8, Java supports functions as first class objects. Lambda expressions of this form are considered of typeFunction<T,U>with T being the domain and U the image type. The expression can be called with its.apply(T t)method, but not with a standard method call. Appleintroducedblocks, a form of closure, as a nonstandard extension intoC,C++,Objective-C 2.0and inMac OS X 10.6 "Snow Leopard"andiOS 4.0. Apple made their implementation available for the GCC and clang compilers. Pointers to block and block literals are marked with^. Normal local variables are captured by value when the block is created, and are read-only inside the block. Variables to be captured by reference are marked with__block. Blocks that need to persist outside of the scope they are created in may need to be copied.[20][21] C#anonymous methods and lambda expressions support closure: Visual Basic .NET, which has many language features similar to those of C#, also supports lambda expressions with closures: InD, closures are implemented by delegates, a function pointer paired with a context pointer (e.g. a class instance, or a stack frame on the heap in the case of closures). D version 1, has limited closure support. For example, the above code will not work correctly, because the variable a is on the stack, and after returning from test(), it is no longer valid to use it (most probably calling foo via dg(), will return a 'random' integer). This can be solved by explicitly allocating the variable 'a' on heap, or using structs or class to store all needed closed variables and construct a delegate from a method implementing the same code. Closures can be passed to other functions, as long as they are only used while the referenced values are still valid (for example calling another function with a closure as a callback parameter), and are useful for writing generic data processing code, so this limitation, in practice, is often not an issue. This limitation was fixed in D version 2 - the variable 'a' will be automatically allocated on the heap because it is used in the inner function, and a delegate of that function can escape the current scope (via assignment to dg or return). Any other local variables (or arguments) that are not referenced by delegates or that are only referenced by delegates that do not escape the current scope, remain on the stack, which is simpler and faster than heap allocation. The same is true for inner's class methods that reference a function's variables. C++enables definingfunction objectsby overloadingoperator(). These objects behave somewhat like functions in a functional programming language. They may be created at runtime and may contain state, but they do not implicitly capture local variables as closures do. As ofthe 2011 revision, the C++ language also supports closures, which are a type of function object constructed automatically from a special language construct calledlambda-expression. A C++ closure may capture its context either by storing copies of the accessed variables as members of the closure object or by reference. In the latter case, if the closure object escapes the scope of a referenced object, invoking itsoperator()causes undefined behavior since C++ closures do not extend the lifetime of their context. Eiffelincludes inline agents defining closures. An inline agent is an object representing a routine, defined by giving the code of the routine in-line. For example, in the argument tosubscribeis an agent, representing a procedure with two arguments; the procedure finds the country at the corresponding coordinates and displays it. The whole agent is "subscribed" to the event typeclick_eventfor a certain button, so that whenever an instance of the event type occurs on that button – because a user has clicked the button – the procedure will be executed with the mouse coordinates being passed as arguments forxandy. The main limitation of Eiffel agents, which distinguishes them from closures in other languages, is that they cannot reference local variables from the enclosing scope. This design decision helps in avoiding ambiguity when talking about a local variable value in a closure - should it be the latest value of the variable or the value captured when the agent is created? OnlyCurrent(a reference to current object, analogous tothisin Java), its features, and arguments of the agent can be accessed from within the agent body. The values of the outer local variables can be passed by providing additional closed operands to the agent. Embarcadero C++Builder provides the reserved word__closureto provide a pointer to a method with a similar syntax to a function pointer.[22] Standard C allows writing atypedeffor a pointer to afunction typeusing the following syntax: In a similar way, atypedefcan be declared for a pointer to a method using this syntax:	https://en.wikipedia.org/wiki/Closure_(computer_science)
Incomputer science, acontinuationis anabstract representationof thecontrol stateof acomputer program. A continuation implements (reifies) the program control state, i.e. the continuation is a data structure that represents the computational process at a given point in the process's execution; the created data structure can be accessed by the programming language, instead of being hidden in theruntime environment. Continuations are useful for encoding other control mechanisms in programming languages such asexceptions,generators,coroutines, and so on. The "current continuation" or "continuation of the computation step" is the continuation that, from the perspective of running code, would be derived from the current point in a program's execution. The termcontinuationscan also be used to refer tofirst-class continuations, which are constructs that give aprogramming languagethe ability to save the execution state at any point and return to that point at a later point in the program, possibly multiple times. The earliest description of continuations was made byAdriaan van Wijngaardenin September 1964. Wijngaarden spoke at the IFIP Working Conference on Formal Language Description Languages held in Baden bei Wien, Austria. As part of a formulation for anAlgol 60preprocessor, he called for a transformation of proper procedures intocontinuation-passing style,[1]though he did not use this name, and his intention was to simplify a program and thus make its result more clear. Christopher Strachey,Christopher P. WadsworthandJohn C. Reynoldsbrought the termcontinuationinto prominence in their work in the field ofdenotational semanticsthat makes extensive use of continuations to allow sequential programs to be analysed in terms offunctional programmingsemantics.[1] Steve Russell[2]invented the continuation in his secondLispimplementation for theIBM 704, though he did not name it.[3] Reynolds (1993)gives a complete history of the discovery of continuations. First-class continuations are a language's ability to completely control the execution order of instructions. They can be used to jump to a function that produced the call to the current function, or to a function that has previously exited. One can think of a first-class continuation as saving theexecutionstate of the program. True first-class continuations do not save program data – unlike aprocess image– only the execution context. This is illustrated by the "continuation sandwich" description: Say you're in the kitchen in front of the refrigerator, thinking about a sandwich. You take a continuation right there and stick it in your pocket. Then you get some turkey and bread out of the refrigerator and make yourself a sandwich, which is now sitting on the counter. You invoke the continuation in your pocket, and you find yourself standing in front of the refrigerator again, thinking about a sandwich. But fortunately, there's a sandwich on the counter, and all the materials used to make it are gone. So you eat it. :-)[4] In this description, the sandwich is part of the programdata(e.g., an object on the heap), and rather than calling a "make sandwich" routine and then returning, the person called a "make sandwich with current continuation" routine, which creates the sandwich and then continues where execution left off. Schemewas the first full production system providing first "catch"[1]and thencall/cc. Bruce Duba introduced call/cc intoSML. Continuations are also used in models of computation includingdenotational semantics, theactor model,process calculi, andlambda calculus. These models rely on programmers or semantics engineers to write mathematical functions in the so-calledcontinuation-passing style. This means that each function consumes a function that represents the rest of the computation relative to this function call. To return a value, the function calls this "continuation function" with a return value; to abort the computation it returns a value. Functional programmers who write their programs incontinuation-passing stylegain the expressive power to manipulate the flow of control in arbitrary ways. The cost is that they must maintain the invariants of control and continuations by hand, which can be a highly complex undertaking (but see 'continuation-passing style' below). Continuations simplify and clarify the implementation of several commondesign patterns, includingcoroutines/green threadsandexception handling, by providing the basic, low-level primitive which unifies these seemingly unconnected patterns. Continuations can provide elegant solutions to some difficult high-level problems, like programming a web server that supports multiple pages, accessed by the use of the forward and back buttons and by following links. TheSmalltalkSeasideweb framework uses continuations to great effect, allowing one to program the web server in procedural style, by switching continuations when switching pages. More complex constructs for which"continuations provide an elegant description"[1]also exist. For example, inC,longjmpcan be used to jump from the middle of onefunctionto another, provided the second function lies deeper in the stack (if it is waiting for the first function to return, possibly among others). Other more complex examples includecoroutinesinSimula 67,Lua, andPerl; tasklets inStackless Python;generatorsinIconandPython; continuations inScala(starting in 2.8);fibersinRuby(starting in 1.9.1); thebacktrackingmechanism inProlog;monadsinfunctional programming; andthreads. TheSchemeprogramming language includes the control operatorcall-with-current-continuation(abbreviated as: call/cc) with which a Scheme program can manipulate the flow of control: Using the above, the following code block defines a functiontestthat setsthe-continuationto the future execution state of itself: For a gentler introduction to this mechanism, seecall-with-current-continuation. This example shows a possible usage of continuations to implementcoroutinesas separate threads.[5] The functions defined above allow for defining and executing threads throughcooperative multitasking, i.e. threads that yield control to the next one in a queue: The previous code will produce this output: A program must allocate space in memory for the variables its functions use. Most programming languages use acall stackfor storing the variables needed because it allows for fast and simple allocating and automatic deallocation of memory. Other programming languages use aheapfor this, which allows for flexibility at a higher cost for allocating and deallocating memory. Both of these implementations have benefits and drawbacks in the context of continuations.[6] Many programming languages exhibit first-class continuations under various names; specifically: In any language which supportsclosuresandproper tail calls, it is possible to write programs incontinuation-passing styleand manually implement call/cc. (In continuation-passing style, call/cc becomes a simple function that can be written withlambda.) This is a particularly common strategy inHaskell, where it is easy to construct a "continuation-passingmonad" (for example, theContmonad andContTmonad transformer in themtllibrary). The support forproper tail callsis needed because in continuation-passing style no function ever returns;allcalls are tail calls. One area that has seen practical use of continuations is inWeb programming.[7][8]The use of continuations shields the programmer from thestatelessnature of theHTTPprotocol. In the traditional model of web programming, the lack of state is reflected in the program's structure, leading to code constructed around a model that lends itself very poorly to expressing computational problems. Thus continuations enable code that has the useful properties associated withinversion of control, while avoiding its problems. "Inverting back the inversion of control or, Continuations versus page-centric programming"[9]is a paper that provides a good introduction to continuations applied to web programming. Support for continuations varies widely. A programming language supportsre-invocablecontinuations if a continuation may be invoked repeatedly (even after it has already returned). Re-invocable continuations were introduced byPeter J. Landinusing hisJ(for Jump) operator that could transfer the flow of control back into the middle of a procedure invocation. Re-invocable continuations have also been called "re-entrant" in theRacketlanguage. However this use of the term "re-entrant" can be easily confused with its use in discussions ofmultithreading. A more limited kind is theescape continuationthat may be used to escape the current context to a surrounding one. Many languages which do not explicitly support continuations supportexception handling, which is equivalent to escape continuations and can be used for the same purposes. C'ssetjmp/longjmpare also equivalent: they can only be used tounwind the stack. Escape continuations can also be used to implementtail call elimination. One generalization of continuations aredelimited continuations. Continuation operators likecall/cccapture theentireremaining computation at a given point in the program and provide no way of delimiting this capture. Delimited continuation operators address this by providing two separate control mechanisms: apromptthat delimits a continuation operation and areificationoperator such asshiftorcontrol. Continuations captured using delimited operators thus only represent a slice of the program context. Continuations are the functional expression of theGOTOstatement, and the same caveats apply.[10]While they are a sensible option in some special cases such as web programming, use of continuations can result in code that is difficult to follow. In fact, theesoteric programming languageUnlambdaincludescall-with-current-continuationas one of its features solely because expressions involving it "tend to be hopelessly difficult to track down".[11]The external links below illustrate the concept in more detail. In "Continuations and the nature of quantification",Chris Barkerintroduced the "continuation hypothesis", that some linguistic expressions (in particular, QNPs [quantificational noun phrases]) have denotations that manipulate their own continuations.[12] Barker argued that this hypothesis could be used to explain phenomena such asduality of NP meaning(e.g., the fact that the QNP "everyone" behaves very differently from the non-quantificational noun phrase "Bob" in contributing towards the meaning of a sentence like "Alice sees [Bob/everyone]"),scope displacement(e.g., that "a raindrop fell on every car" is interpreted typically as∀c∃r,fell(r,c){\displaystyle \forall c\exists r,{\mbox{fell}}(r,c)}rather than as∃r∀c,fell(r,c){\displaystyle \exists r\forall c,{\mbox{fell}}(r,c)}), andscope ambiguity(that a sentence like "someone saw everyone" may be ambiguous between∃x∀y,saw(x,y){\displaystyle \exists x\forall y,{\mbox{saw}}(x,y)}and∀y∃x,saw(x,y){\displaystyle \forall y\exists x,{\mbox{saw}}(x,y)}). He also observed that this idea is in a way just a natural extension ofRichard Montague's approachin "The Proper Treatment of Quantification in Ordinary English" (PTQ), writing that "with the benefit of hindsight, a limited form of continuation-passing is clearly discernible at the core of Montague’s (1973) PTQ treatment of NPs as generalized quantifiers". The extent to which continuations can be used to explain other general phenomena in natural language is a topic of current research.[13]	https://en.wikipedia.org/wiki/Continuation
Adelegateis a form oftype-safefunction pointerused by theCommon Language Infrastructure(CLI). Delegates specify amethodto call and optionally anobjectto call the method on. Delegates are used, among other things, to implementcallbacksandevent listeners. A delegate object encapsulates a reference to a method. The delegate object can then be passed to code that can call thereferencedmethod, without having to know at compile time which method will be invoked. Amulticast delegateis a delegate that points to several methods.[1][2]Multicastdelegation is a mechanism that provides functionality to execute more than one method. There is a list of delegates maintained internally, and when the multicast delegate is invoked, the list of delegates is executed. In C#, delegates are often used to implement callbacks in event driven programming. For example, a delegate may be used to indicate which method should be called when the user clicks on some button. Delegates allow the programmer to notify several methods that an event has occurred.[3] Code to declare adelegatetype, namedSendMessageDelegate, which takes aMessageas a parameter and returnsvoid: Code to define a method that takes an instantiated delegate as its argument: The implemented method that runs when the delegate is called: Code to call the SendMessage method, passing an instantiated delegate as an argument: A delegate variable calls the associated method and is called as follows: Delegate variables arefirst-class objectsof the formnewDelegateType(obj.Method)and can be assigned to any matching method, or to the valuenull. They store a methodandits receiver without any parameters:[4] The objectfunnyObjcan bethisand omitted. If the method isstatic, it should not be the object (also called an instance in other languages), but the class itself. It should not beabstract, but could benew,overrideorvirtual. To call a method with a delegate successfully, the method signature has to match theDelegateTypewith the same number of parameters of the same kind (ref,out,value) with the same type (including return type). A delegate variable can hold multiple values at the same time: If the multicast delegate is a function or has nooutparameter, the parameter of the last call is returned.[5] Although internalimplementationsmay vary, delegateinstancescan be thought of as atupleof anobjectand amethodpointerand areference(possibly null) to another delegate. Hence a reference to one delegate is possibly a reference to multiple delegates. When the first delegate has finished, if its chain reference is not null, the next will be invoked, and so on until the list is complete. This pattern allows aneventto have overhead scaling easily from that of a single reference up to dispatch to a list of delegates, and is widely used in the CLI. Performance of delegates used to be much slower than avirtualorinterfacemethod call (6 to 8 times slower in Microsoft's 2003 benchmarks),[6]but, since the.NET2.0CLRin 2005, it is about the same as interface calls.[7]This means there is a small added overhead compared to direct method invocations. There are very stringent rules on the construction of delegate classes. These rules permit optimizing compilers a great deal of leeway when optimizing delegates while ensuring type safety.[citation needed]	https://en.wikipedia.org/wiki/Delegate_(CLI)
Inobject-oriented design, thedependency inversion principleis a specific methodology forloosely coupledsoftwaremodules. When following this principle, the conventionaldependencyrelationships established from high-level, policy-setting modules to low-level, dependency modules are reversed, thus rendering high-level modules independent of the low-level module implementation details. The principle states:[1] By dictating thatbothhigh-level and low-level objects must depend on the same abstraction, this design principleinvertsthe way some people may think about object-oriented programming.[2] The idea behind points A and B of this principle is that when designing the interaction between a high-level module and a low-level one, the interaction should be thought of as an abstract interaction between them. This has implications for the design of both the high-level and the low-level modules: the low-level one should be designed with the interaction in mind and it may be necessary to change its usage interface. In many cases, thinking about the interaction itself as an abstract concept allows for reduction of the coupling between the components without introducing additional coding patterns and results in a lighter and less implementation-dependent interaction schema. When this abstract interaction schema is generic and clear, this design principle leads to the dependency inversion pattern described below. In conventional application architecture, lower-level components (e.g., Utility Layer) are designed to be consumed by higher-level components (e.g., Policy Layer) which enable highly composite systems to be built. Here higher-level components depend directly upon lower-level components to achieve some task. This dependency upon lower-level components limits the reuse opportunities of the higher-level components.[1] The goal of the dependency inversion pattern is to avoid this highly coupled distribution with the mediation of an abstract layer, and to increase the reusability of higher and policy layers. With the introduction of an abstract layer, both high- and lower-level layers reduce the traditional dependencies from top to bottom. Nevertheless, the "inversion" concept does not mean that lower-level layersdirectlydepend on higher-level layers. Rather,bothlayers should depend on abstractions (interfaces) that expose the behavior needed by higher-level layers. In a direct application of dependency inversion, the abstracts are owned by the higher/policy layers. This architecture groups the higher/policy components and the abstractions that define lower services together in the same package. The lower-level layers are created by inheritance/implementation of theseabstract classesor interfaces.[1] The inversion of the dependencies and ownership encourages reuse of the higher and policy layers. Upper layers could use other implementations of the lower services. When the lower-level layer components are closed or when the application requires the reuse of existing services, it is common that anAdaptermediates between the services and the abstractions. In many projects the dependency inversion principle and pattern are considered as a single concept that should be generalized, i.e., applied to all interfaces between software modules. There are at least two reasons for that: If the mocking tool used relies only on inheritance, it may become necessary to widely apply the dependency inversion pattern. This has major drawbacks: The presence of interfaces to accomplish the Dependency Inversion Pattern (DIP) has other design implications in anobject-oriented program: Using inheritance-based mocking tools also introduces restrictions: Two common implementations of DIP use similar logical architecture but with different implications. A direct implementation packages the policy classes with service abstracts classes in one library. In this implementation high-level components and low-level components are distributed into separate packages/libraries, where theinterfacesdefining the behavior/services required by the high-level component are owned by, and exist within the high-level component's library. The implementation of the high-level component's interface by the low-level component requires that the low-level component package depend upon the high-level component for compilation, thus inverting the conventional dependency relationship. Figures 1 and 2 illustrate code with the same functionality, however in Figure 2, an interface has been used to invert the dependency. The direction of dependency can be chosen to maximize policy code reuse, and eliminate cyclic dependencies. In this version of DIP, the lower layer component's dependency on the interfaces/abstracts in the higher-level layers makes reuse of the lower layer components difficult. This implementation instead "inverts" the traditional dependency from top-to-bottom to the opposite, bottom-to-top. A more flexible solution extracts the abstract components into an independent set of packages/libraries: The separation of each layer into its own package encourages reuse of any layer, providing robustness and mobility.[1] A genealogical system may represent relationships between people as a graph of direct relationships between them (father-son, father-daughter, mother-son, mother-daughter, husband-wife, wife-husband, etc.). This is very efficient and extensible, as it is easy to add an ex-husband or a legal guardian. But some higher-level modules may require a simpler way to browse the system: any person may have children, parents, siblings (including half-brothers and -sisters or not), grandparents, cousins, and so on. Depending on the usage of the genealogical module, presenting common relationships as distinct direct properties (hiding the graph) will make the coupling between a higher-level module and the genealogical module much lighter and allow changing the internal representation of the direct relationships completely without any effect on the modules using them. It also permits embedding exact definitions of siblings or uncles in the genealogical module, thus enforcing thesingle responsibility principle. Finally, if the first extensible generalized graph approach seems the most extensible, the usage of the genealogical module may show that a more specialized and simpler relationship implementation is sufficient for the application(s) and helps create a more efficient system. In this example, abstracting the interaction between the modules leads to a simplified interface of the lower-level module and may lead to a simpler implementation of it. A remote file server (FTP, cloud storage ...) client can be modeled as a set of abstract interfaces: If local and remote files offers the same abstract interfaces, high-level modules that implement the dependency inversion pattern can use them indiscriminately. The application will be able to save its documents locally or remotely transparently. The level of service required by high level modules should be considered. Designing a module as a set of abstract interfaces, and adapting other modules to it, can provide a common interface for many systems. UI and ApplicationLayer packages contain mainly concrete classes. Controllers contains abstracts/interface types. UI has an instance of ICustomerHandler. All packages are physically separated. In the ApplicationLayer there is a concrete implementation of CustomerHandler that Page class will use. Instances of the ICustomerHandler interface are created dynamically by a Factory (possibly in the same Controllers package). Concrete types Page and CustomerHandler depend on ICustomerHandler, not on each other. Since the UI doesn't reference the ApplicationLayer or any other concrete package implementing ICustomerHandler, the concrete implementation of CustomerHandler can be replaced without changing the UI class. Also, the Page class implements interface IPageViewer which could be passed as an argument to ICustomerHandler methods, allowing the concrete implementation of CustomerHandler to communicate with UI without a concrete dependency. Again, both are linked by interfaces. Applying the dependency inversion principle can also be seen as an example of theadapter pattern. That is, the high-level class defines its own adapter interface which is the abstraction on which the other high-level classes depend. The adapted implementation also depends necessarily on the same adapter interface abstraction, while it can be implemented by using code from within its own low-level module. The high-level module does not depend on the low-level module, since it only uses the low-level functionality indirectly through the adapter interface by invoking polymorphic methods to the interface which are implemented by the adapted implementation and its low-level module.[citation needed] Various patterns such asPlugin,Service Locator,[3]orDependency injection[4][5]are employed to facilitate the run-time provisioning of the chosen low-level component implementation to the high-level component.[citation needed] The dependency inversion principle was postulated byRobert C. Martinand described in several publications including the paperObject Oriented Design Quality Metrics: an analysis of dependencies,[6]an article appearing in the C++ Report in June 1996 entitledThe Dependency Inversion Principle,[7]and the booksAgile Software Development, Principles, Patterns, and Practices,[1]andAgile Principles, Patterns, and Practices in C#.	https://en.wikipedia.org/wiki/Dependency_inversion_principle
Incomputer programming,flow-based programming(FBP) is aprogramming paradigmthat definesapplicationsas networks ofblack boxprocesses, which exchange data across predefined connections bymessage passing, where the connections are specifiedexternallyto the processes. These black box processes can be reconnected endlessly to form different applications without having to be changed internally. FBP is thus naturallycomponent-oriented. FBP is a particular form ofdataflow programmingbased on bounded buffers, information packets with defined lifetimes, named ports, and separate definition of connections. Flow-based programming defines applications using the metaphor of a "data factory". It views an application not as a single, sequential process, which starts at a point in time, and then does one thing at a time until it is finished, but as a network of asynchronous processes communicating by means ofstreamsof structured data chunks, called "information packets" (IPs). In this view, the focus is on the application data and the transformations applied to it to produce the desired outputs. The network is defined externally to the processes, as a list of connections which is interpreted by a piece of software, usually called the "scheduler". The processes communicate by means of fixed-capacity connections. A connection is attached to a process by means of aport, which has a name agreed upon between the process code and the network definition. More than one process can execute the same piece of code. At any point in time, a given IP can only be "owned" by a single process, or be in transit between two processes.Portsmay either be simple, or array-type, as used e.g. for the input port of the Collate component described below. It is the combination of ports with asynchronous processes that allows many long-running primitive functions of data processing, such as Sort, Merge, Summarize, etc., to be supported in the form of softwareblack boxes. Because FBP processes can continue executing as long they have data to work on and somewhere to put their output, FBP applications generally run in less elapsed time than conventional programs, and make optimal use of all the processors on a machine, with no special programming required to achieve this.[1] The network definition is usually diagrammatic, and is converted into a connection list in some lower-level language or notation. FBP is often avisual programming languageat this level. More complex network definitions have a hierarchical structure, being built up from subnets with "sticky" connections. Many other flow-based languages/runtimes are built around more traditional programming languages, the most notable[citation needed]example isRaftLibwhich uses C++ iostream-like operators to specify the flow graph. FBP has much in common with theLinda[2]language in that it is, inGelernterand Carriero's terminology, a "coordination language":[3]it is essentially language-independent. Indeed, given a scheduler written in a sufficiently low-level language, components written in different languages can be linked together in a single network. FBP thus lends itself to the concept ofdomain-specific languagesor "mini-languages". FBP exhibits "data coupling", described in the article oncouplingas the loosest type of coupling between components. The concept ofloose couplingis in turn related to that ofservice-oriented architectures, and FBP fits a number of the criteria for such an architecture, albeit at a more fine-grained level than most examples of this architecture. FBP promotes high-level, functional style of specifications that simplify reasoning about system behavior. An example of this is thedistributed data flowmodel for constructively specifying and analyzing the semantics of distributed multi-party protocols. Flow-based programming was invented byJ. Paul Morrisonin the early 1970s, and initially implemented in software for a Canadian bank.[4]FBP at its inception was strongly influenced by some IBM simulation languages of the period, in particularGPSS, but its roots go all the way back toConway's seminal paper on what he calledcoroutines.[5] FBP has undergone a number of name changes over the years: the original implementation was called AMPS (Advanced Modular Processing System). One large application in Canada went live in 1975, and, as of 2013, has been in continuous production use, running daily, for almost 40 years. Because IBM considered the ideas behind FBP "too much like a law of nature" to be patentable they instead put the basic concepts of FBP into the public domain, by means of aTechnical Disclosure Bulletin, "Data Responsive Modular, Interleaved Task Programming System",[6]in 1971.[4]An article describing its concepts and experience using it was published in 1978 in theIBM ResearchIBM Systems Journal under the name DSLM.[7]A second implementation was done as a joint project of IBM Canada and IBM Japan, under the name "Data Flow Development Manager" (DFDM), and was briefly marketed in Japan in the late '80s under the name "Data Flow Programming Manager". Generally the concepts were referred to within IBM as "Data Flow", but this term was felt to be too general, and eventually the name "Flow-Based Programming" was adopted. From the early '80s to 1993 J. Paul Morrison and IBM architectWayne Stevensrefined and promoted the concepts behind FBP. Stevens wrote several articles describing and supporting the FBP concept, and included material about it in several of his books.[8][9][non-primary source needed][10][non-primary source needed]. In 1994 Morrison published a book describing FBP, and providing empirical evidence that FBP led to reduced development times.[11] The following diagram shows the major entities of an FBP diagram (apart from the Information Packets). Such a diagram can be converted directly into a list of connections, which can then be executed by an appropriate engine (software or hardware). A, B and C are processes executing code components. O1, O2, and the two INs are ports connecting the connections M and N to their respective processes. It is permitted for processes B and C to be executing the same code, so each process must have its own set of working storage, control blocks, etc. Whether or not they do share code, B and C are free to use the same port names, as port names only have meaning within the components referencing them (and at the network level, of course). M and N are what are often referred to as "bounded buffers", and have a fixed capacity in terms of the number of IPs that they can hold at any point in time. The concept ofportsis what allows the same component to be used at more than one place in the network. In combination with a parametrization ability, called Initial Information Packets (IIPs), ports provide FBP with a component reuse ability, making FBP acomponent-basedarchitecture. FBP thus exhibits what Raoul de Campo andNate EdwardsofIBM Researchhave termedconfigurable modularity. Information Packets or IPs are allocated in what might be called "IP space" (just as Linda's tuples are allocated in "tuple space"), and have a well-defined lifetime until they are disposed of and their space is reclaimed - in FBP this must be an explicit action on the part of an owning process. IPs traveling across a given connection (actually it is their "handles" that travel) constitute a "stream", which is generated and consumed asynchronously - this concept thus has similarities to thelazy consconcept described in the 1976 article by Friedman and Wise.[12] IPs are usually structured chunks of data - some IPs, however, may not contain any real data, but are used simply as signals. An example of this is "bracket IPs", which can be used to group data IPs into sequential patterns within a stream, called "substreams". Substreams may in turn be nested. IPs may also be chained together to form "IP trees", which travel through the network as single objects. The system of connections and processes described above can be "ramified" to any size. During the development of an application, monitoring processes may be added between pairs of processes, processes may be "exploded" to subnets, or simulations of processes may be replaced by the real process logic. FBP therefore lends itself torapid prototyping. This is really anassembly lineimage of data processing: the IPs travelling through a network of processes may be thought of as widgets travelling from station to station in an assembly line. "Machines" may easily be reconnected, taken off line for repair, replaced, and so on. Oddly enough, this image is very similar to that ofunit record equipmentthat was used to process data before the days of computers, except that decks of cards had to be hand-carried from one machine to another. Implementations of FBP may be non-preemptive or preemptive - the earlier implementations tended to be non-preemptive (mainframe and C language), whereas the latest Java implementation (see below) uses Java Thread class and is preemptive. FBP components often form complementary pairs. This example uses two such pairs. The problem described seems very simple as described in words, but in fact is surprisingly difficult to accomplish using conventional procedural logic. The task, called the "telegram problem", originally described byPeter Naur, is to write a program which accepts lines of text and generates output lines containing as many words as possible, where the number of characters in each line does not exceed a certain length. The words may not be split and we assume no word is longer than the size of the output lines. This is analogous to the word-wrapping problem in text editors.[13] In conventional logic, the programmer rapidly discovers that neither the input nor the output structures can be used to drive the call hierarchy ofcontrol flow. In FBP, on the other hand, the problem description itself suggests a solution: Here is the most natural solution in FBP (there is no single "correct" solution in FBP, but this seems like a natural fit): where DC and RC stand for "DeCompose" and "ReCompose", respectively. As mentioned above, Initial Information Packets (IIPs) can be used to specify parametric information such as the desired output record length (required by the rightmost two components), or file names. IIPs are data chunks associated with a port in the network definition which become "normal" IPs when a "receive" is issued for the relevant port. This type of program involves passing a file of "details" (changes, adds and deletes) against a "master file", and producing (at least) an updated master file, and one or more reports. Update programs are generally quite hard to code using synchronous, procedural code, as two (sometimes more) input streams have to be kept synchronized, even though there may be masters without corresponding details, or vice versa. In FBP, a reusable component (Collate), based on theunit recordidea of a Collator, makes writing this type of application much easier as Collate merges the two streams and inserts bracket IPs to indicate grouping levels, significantly simplifying the downstream logic. Suppose that one stream ("masters" in this case) consists of IPs with key values of 1, 2 and 3, and the second stream IPs ("details") have key values of 11, 12, 21, 31, 32, 33 and 41, where the first digit corresponds to the master key values. Using bracket characters to represent "bracket" IPs, the collated output stream will be as follows: As there was no master with a value of 4, the last group consists of a single detail (plus brackets). The structure of the above stream can be described succinctly using aBNF-like notation such as Collate is a reusableblack boxwhich only needs to know where the control fields are in its incoming IPs (even this is not strictly necessary as transformer processes can be inserted upstream to place the control fields in standard locations), and can in fact be generalized to any number of input streams, and any depth of bracket nesting. Collate uses an array-type port for input, allowing a variable number of input streams. Flow-based programming supports process multiplexing in a very natural way. Since components are read-only, any number of instances of a given component ("processes") can run asynchronously with each other. When computers usually had a single processor, this was useful when a lot of I/O was going on; now that machines usually have multiple processors, this is starting to become useful when processes are CPU-intensive as well. The diagram in this section shows a single "Load Balancer" process distributing data between three processes, labeled S1, S2 and S3, respectively, which are instances of a single component, which in turn feed into a single process on a "first-come, first served" basis. In this general schematic, requests (transactions) coming from users enter the diagram at the upper left, and responses are returned at the lower left. The "back ends" (on the right side) communicate with systems at other sites, e.g. usingCORBA,MQSeries, etc. The cross-connections represent requests that do not need to go to the back ends, or requests that have to cycle through the network more than once before being returned to the user. As different requests may use different back-ends, and may require differing amounts of time for the back-ends (if used) to process them, provision must be made to relate returned data to the appropriate requesting transactions, e.g.hash tablesor caches. The above diagram is schematic in the sense that the final application may contain many more processes: processes may be inserted between other processes to manage caches, display connection traffic, monitor throughput, etc. Also the blocks in the diagram may represent "subnets" - small networks with one or more open connections. This methodology assumes that a program must be structured as a single procedural hierarchy of subroutines. Its starting point is to describe the application as a set of "main lines", based on the input and output data structures. One of these "main lines" is then chosen to drive the whole program, and the others are required to be "inverted" to turn them into subroutines (hence the name "Jackson inversion"). This sometimes results in what is called a "clash", requiring the program to be split into multiple programs or coroutines. When using FBP, this inversion process is not required, as every FBP component can be considered a separate "main line". FBP and JSP share the concept of treating a program (or some components) as aparserof an input stream. In Jackson's later work,Jackson System Development(JSD), the ideas were developed further.[14][15] In JSD the design is maintained as a network design until the final implementation stage. The model is then transformed into a set of sequential processes to the number of available processors. Jackson discusses the possibility of directly executing the network model that exists prior to this step, in section 1.3 of his book (italics added): FBP was recognized by M A Jackson as an approach that follows his method of "Program decomposition into sequential processes communicating by a coroutine-like mechanism"[16] W.B. Ackerman defines an applicative language as one which does all of its processing by means of operators applied to values.[17]The earliest known applicative language was LISP. An FBP component can be regarded as a function transforming its input stream(s) into its output stream(s). These functions are then combined to make more complex transformations, as shown here: If we label streams, as shown, with lower case letters, then the above diagram can be represented succinctly as follows: Just as in functional notation F can be used twice because it only works with values, and therefore has no side effects, in FBP two instances of a given component may be running concurrently with each other, and therefore FBP components must not have side-effects either. Functional notation could clearly be used to represent at least a part of an FBP network. The question then arises whether FBP components can themselves be expressed using functional notation. W.H. Burge showed how stream expressions can be developed using a recursive, applicative style of programming, but this work was in terms of (streams of) atomic values.[18]In FBP, it is necessary to be able to describe and process structured data chunks (FBP IPs). Furthermore, most applicative systems assume that all the data is available in memory at the same time, whereas FBP applications need to be able to process long-running streams of data while still using finite resources. Friedman and Wise suggested a way to do this by adding the concept of"lazy cons"to Burge's work. This removed the requirement that both of the arguments of "cons" be available at the same instant of time. "Lazy cons" does not actually build a stream until both of its arguments are realized - before that it simply records a "promise" to do this. This allows a stream to be dynamically realized from the front, but with an unrealized back end. The end of the stream stays unrealized until the very end of the process, while the beginning is an ever-lengthening sequence of items. Many of the concepts in FBP seem to have been discovered independently in different systems over the years. Linda, mentioned above, is one such. The difference between the two techniques is illustrated by the Linda "school of piranhas"load balancingtechnique - in FBP, this requires an extra "load balancer" component which routes requests to the component in a list which has the smallest number of IPs waiting to be processed. Clearly FBP and Linda are closely related, and one could easily be used to simulate the other. An object inOOPcan be described as a semi-autonomous unit comprising both information and behaviour. Objects communicate by means of "method calls", which are essentially subroutine calls, done indirectly via the class to which the receiving object belongs. The object's internal data can only be accessed by means of method calls, so this is a form ofinformation hidingor "encapsulation". Encapsulation, however, predates OOP -David Parnaswrote one of the seminal articles on it in the early 70s[19]- and is a basic concept in computing. Encapsulation is the very essence of an FBP component, which may be thought of as ablack box, performing some conversion of its input data into its output data. In FBP, part of the specification of a component is the data formats and stream structures that it can accept, and those it will generate. This constitutes a form ofdesign by contract. In addition, the data in an IP can only be accessed directly by the currently owning process. Encapsulation can also be implemented at the network level, by having outer processes protect inner ones. A paper by C. Ellis and S. Gibbs distinguishes betweenactive objectsand passive objects.[20]Passive objects comprise information and behaviour, as stated above, but they cannot determine thetimingof this behaviour. Active objects on the other hand can do this. In their article Ellis and Gibbs state that active objects have much more potential for the development of maintainable systems than do passive objects. An FBP application can be viewed as a combination of these two types of object, where FBP processes would correspond to active objects, while IPs would correspond to passive objects. FBP considersCarl Hewitt'sactoras an asynchronous processes with 2 ports: one for input messages and one for control signals. A control signal is emitted by the actor itself after each round of execution. The purpose of this signal is to avoid parallel execution of the actor's body and so to allow to access the fields of the actor object without synchronization.	https://en.wikipedia.org/wiki/Flow-based_programming
Implicit invocationis a term used by some authors for a style ofsoftware architecturein which a system is structured aroundevent handling, using a form ofcallback. It is closely related toinversion of controland what is known informally as theHollywood principle. The idea behind implicit invocation is that instead of invoking a procedure directly, a component can announce (or broadcast) one or more events. Other components in the system can register an interest in an event by associating a procedure with the event. When the event is announced the system itself invokes all of the procedures that have been registered for the event. Thus an event announcementimplicitlycauses the invocation of procedures in other modules. Implicit invocation is the core technique behind theobserver pattern.[citation needed] Thiscomputer sciencearticle is astub. You can help Wikipedia byexpanding it.	https://en.wikipedia.org/wiki/Implicit_invocation
In computersystems programming, aninterrupt handler, also known as aninterrupt service routine(ISR), is a special block of code associated with a specificinterruptcondition. Interrupt handlers are initiated by hardware interrupts, software interrupt instructions, or softwareexceptions, and are used for implementingdevice driversor transitions between protected modes of operation, such assystem calls. The traditional form of interrupt handler is the hardware interrupt handler. Hardware interrupts arise from electrical conditions or low-level protocols implemented indigital logic, are usually dispatched via a hard-coded table of interrupt vectors, asynchronously to the normal execution stream (as interrupt masking levels permit), often using a separate stack, and automatically entering into a different execution context (privilege level) for the duration of the interrupt handler's execution. In general, hardware interrupts and their handlers are used to handle high-priority conditions that require the interruption of the current code theprocessoris executing.[1][2] Later it was found convenient for software to be able to trigger the same mechanism by means of a software interrupt (a form of synchronous interrupt). Rather than using a hard-coded interrupt dispatch table at the hardware level, software interrupts are often implemented at theoperating systemlevel as a form ofcallback function. Interrupt handlers have a multitude of functions, which vary based on what triggered the interrupt and the speed at which the interrupt handler completes its task. For example, pressing a key on acomputer keyboard,[1]or moving themouse, triggers interrupts that call interrupt handlers which read the key, or the mouse's position, and copy the associated information into the computer's memory.[2] An interrupt handler is a low-level counterpart ofevent handlers. However, interrupt handlers have an unusual execution context, many harsh constraints in time and space, and their intrinsically asynchronous nature makes them notoriously difficult to debug by standard practice (reproducible test cases generally don't exist), thus demanding a specialized skillset—an important subset ofsystem programming—of software engineers who engage at the hardware interrupt layer. Unlike other event handlers, interrupt handlers are expected to set interrupt flags to appropriate values as part of their core functionality. Even in a CPU which supports nested interrupts, a handler is often reached with all interrupts globally masked by a CPU hardware operation. In this architecture, an interrupt handler would normally save the smallest amount of context necessary, and then reset the global interrupt disable flag at the first opportunity, to permit higher priority interrupts to interrupt the current handler. It is also important for the interrupt handler to quell the current interrupt source by some method (often toggling a flag bit of some kind in a peripheral register) so that the current interrupt isn't immediately repeated on handler exit, resulting in an infinite loop. Exiting an interrupt handler with the interrupt system in exactly the right state under every eventuality can sometimes be an arduous and exacting task, and its mishandling is the source of many serious bugs, of the kind that halt the system completely. These bugs are sometimes intermittent, with the mishandled edge case not occurring for weeks or months of continuous operation. Formal validation of interrupt handlers is tremendously difficult, while testing typically identifies only the most frequent failure modes, thus subtle, intermittent bugs in interrupt handlers often ship to end customers. In a modern operating system, upon entry the execution context of a hardware interrupt handler is subtle. For reasons of performance, the handler will typically be initiated in the memory and execution context of the running process, to which it has no special connection (the interrupt is essentially usurping the running context—process time accounting will often accrue time spent handling interrupts to the interrupted process). However, unlike the interrupted process, the interrupt is usually elevated by a hard-coded CPU mechanism to a privilege level high enough to access hardware resources directly. In a low-level microcontroller, the chip might lack protection modes and have nomemory management unit(MMU). In these chips, the execution context of an interrupt handler will be essentially the same as the interrupted program, which typically runs on a small stack of fixed size (memory resources have traditionally been extremely scant at the low end). Nested interrupts are often provided, which exacerbates stack usage. A primary constraint on the interrupt handler in this programming endeavour is to not exceed the available stack in the worst-case condition, requiring the programmer to reason globally about the stack space requirement of every implemented interrupt handler and application task. When allocated stack space is exceeded (a condition known as astack overflow), this is not normally detected in hardware by chips of this class. If the stack is exceeded into another writable memory area, the handler will typically work as expected, but the application will fail later (sometimes much later) due to the handler's side effect of memory corruption. If the stack is exceeded into a non-writable (or protected) memory area, the failure will usually occur inside the handler itself (generally the easier case to later debug). In the writable case, one can implement a sentinel stack guard—a fixed value right beyond the end of the legal stack whose valuecanbe overwritten, but never will be if the system operates correctly. It is common to regularly observe corruption of the stack guard with some kind of watch dog mechanism. This will catch the majority of stack overflow conditions at a point in time close to the offending operation. In a multitasking system, each thread of execution will typically have its own stack. If no special system stack is provided for interrupts, interrupts will consume stack space from whatever thread of execution is interrupted. These designs usually contain an MMU, and the user stacks are usually configured such that stack overflow is trapped by the MMU, either as a system error (for debugging) or to remap memory to extend the space available. Memory resources at this level of microcontroller are typically far less constrained, so that stacks can be allocated with a generous safety margin. In systems supporting high thread counts, it is better if the hardware interrupt mechanism switches the stack to a special system stack, so that none of the thread stacks need account for worst-case nested interrupt usage. Tiny CPUs as far back as the 8-bitMotorola 6809from 1978 have provided separate system and user stack pointers. For many reasons, it is highly desired that the interrupt handler execute as briefly as possible, and it is highly discouraged (or forbidden) for a hardware interrupt to invoke potentially blocking system calls. In a system with multiple execution cores, considerations ofreentrancyare also paramount. If the system provides for hardwareDMA,concurrencyissues can arise even with only a single CPU core. (It is not uncommon for a mid-tier microcontroller to lack protection levels and an MMU, but still provide a DMA engine with many channels; in this scenario, many interrupts are typicallytriggeredby the DMA engine itself, and the associated interrupt handler is expected to tread carefully.) A modern practice has evolved to divide hardware interrupt handlers into front-half and back-half elements. The front-half (or first level) receives the initial interrupt in the context of the running process, does the minimal work to restore the hardware to a less urgent condition (such as emptying a full receive buffer) and then marks the back-half (or second level) for execution in the near future at the appropriate scheduling priority; once invoked, the back-half operates in its own process context with fewer restrictions and completes the handler's logical operation (such as conveying the newly received data to an operating system data queue). In several operating systems‍—‌Linux,Unix,[citation needed]macOS,Microsoft Windows,z/OS,DESQviewand some other operating systems used in the past‍—‌interrupt handlers are divided into two parts: theFirst-Level Interrupt Handler(FLIH) and theSecond-Level Interrupt Handlers(SLIH). FLIHs are also known ashard interrupt handlersorfast interrupt handlers, and SLIHs are also known asslow/soft interrupt handlers, orDeferred Procedure Callsin Windows. A FLIH implements at minimum platform-specific interrupt handling similar tointerrupt routines. In response to an interrupt, there is acontext switch, and the code for the interrupt is loaded and executed. The job of a FLIH is to quickly service the interrupt, or to record platform-specific critical information which is only available at the time of the interrupt, andschedulethe execution of a SLIH for further long-lived interrupt handling.[2] FLIHs causejitterin process execution. FLIHs also mask interrupts. Reducing the jitter is most important forreal-time operating systems, since they must maintain a guarantee that execution of specific code will complete within an agreed amount of time. To reduce jitter and to reduce the potential for losing data from masked interrupts, programmers attempt to minimize the execution time of a FLIH, moving as much as possible to the SLIH. With the speed of modern computers, FLIHs may implement all device and platform-dependent handling, and use a SLIH for further platform-independent long-lived handling. FLIHs which service hardware typically mask their associated interrupt (or keep it masked as the case may be) until they complete their execution. An (unusual) FLIH which unmasks its associated interrupt before it completes is called areentrant interrupt handler. Reentrant interrupt handlers might cause astack overflowfrom multiplepreemptionsby the sameinterrupt vector, and so they are usually avoided. In apriority interruptsystem, the FLIH also (briefly) masks other interrupts of equal or lesser priority. A SLIH completes long interrupt processing tasks similarly to a process. SLIHs either have a dedicatedkernelthread for each handler, or are executed by a pool of kernel worker threads. These threads sit on arun queuein the operating system until processor time is available for them to perform processing for the interrupt. SLIHs may have a long-lived execution time, and thus are typically scheduled similarly to threads and processes. In Linux, FLIHs are calledupper half, and SLIHs are calledlower halforbottom half.[1][2]This is different from naming used in other Unix-like systems, where both are a part ofbottom half.[clarification needed]	https://en.wikipedia.org/wiki/Interrupt_handler
Incomputer science,message passingis a technique for invoking behavior (i.e., running aprogram) on acomputer. The invoking program sends a message to aprocess(which may be anactororobject) and relies on that process and its supporting infrastructure to then select and run some appropriate code. Message passing differs from conventional programming where a process,subroutine, or function is directly invoked by name. Message passing is key to somemodels of concurrencyandobject-oriented programming. Message passing is ubiquitous in modern computersoftware.[citation needed]It is used as a way for the objects that make up a program to work with each other and as a means for objects and systems running on different computers (e.g., theInternet) to interact. Message passing may be implemented by various mechanisms, includingchannels. Message passing is a technique for invoking behavior (i.e., running a program) on a computer. In contrast to the traditional technique of calling a program by name, message passing uses anobject modelto distinguish the general function from the specific implementations. The invoking program sends a message and relies on the object to select and execute the appropriate code. The justifications for using an intermediate layer essentially falls into two categories: encapsulation and distribution. Encapsulationis the idea that software objects should be able to invoke services on other objects without knowing or caring about how those services are implemented. Encapsulation can reduce the amount of coding logic and make systems more maintainable. E.g., rather than having IF-THEN statements that determine which subroutine or function to call, a developer can just send a message to the object and the object will select the appropriate code based on its type. One of the first examples of how this can be used was in the domain of computer graphics. There are various complexities involved in manipulating graphic objects. For example, simply using the right formula to compute the area of an enclosed shape will vary depending on if the shape is a triangle, rectangle, ellipse, or circle. In traditional computer programming, this would result in long IF-THEN statements testing what sort of object the shape was and calling the appropriate code. The object-oriented way to handle this is to define a class calledShapewith subclasses such asRectangleandEllipse(which, in turn, have subclassesSquareandCircle) and then to simply send a message to anyShapeasking it to compute its area. EachShapeobject will then invoke the subclass's method with the formula appropriate for that kind of object.[1] Distributed message passing provides developers with a layer of the architecture that provides common services to build systems made up of sub-systems that run on disparate computers in different locations and at different times. When a distributed object is sending a message, the messaging layer can take care of issues such as: Synchronous message passing occurs between objects that are running at the same time. It is used by object-oriented programming languages such asJavaandSmalltalk. Synchronous messaging is analogous to a synchronous function call; just as the function caller waits until the function completes, the sending process waits until the receiving process completes. This can make synchronous communication unworkable for some applications. For example, large, distributed systems may not perform well enough to be usable. Such large, distributed systems may need to operate while some of their subsystems are down for maintenance, etc. Imagine a busy business office having 100 desktop computers that send emails to each other using synchronous message passing exclusively. One worker turning off their computer can cause the other 99 computers to freeze until the worker turns their computer back on to process a single email. With asynchronous message passing the receiving object can be down or busy when the requesting object sends the message. Continuing the function call analogy, it is like a function call that returns immediately, without waiting for the called function to complete. Messages are sent to a queue where they are stored until the receiving process requests them. The receiving process processes its messages and sends results to a queue for pickup by the original process (or some designated next process).[3] Asynchronous messaging requires additional capabilities for storing and retransmitting data for systems that may not run concurrently, and are generally handled by an intermediary level of software (often calledmiddleware); a common type being Message-oriented middleware (MOM). The buffer required inasynchronous communicationcan cause problems when it is full. A decision has to be made whether to block the sender or whether to discard future messages. A blocked sender may lead todeadlock. If messages are dropped, communication is no longer reliable. Synchronous communication can be built on top of asynchronous communication by using aSynchronizer. For example, the α-Synchronizer works by ensuring that the sender always waits for an acknowledgement message from the receiver. The sender only sends the next message after the acknowledgement has been received. On the other hand, asynchronous communication can also be built on top of synchronous communication. For example, modernmicrokernelsgenerally only provide asynchronous messagingprimitive[citation needed]and asynchronous messaging can be implemented on top by usinghelper threads. Message-passing systems use either distributed or local objects. With distributed objects the sender and receiver may be on different computers, running different operating systems, using different programming languages, etc. In this case the bus layer takes care of details about converting data from one system to another, sending and receiving data across the network, etc. TheRemote Procedure Call(RPC) protocol inUnixwas an early example of this. With this type of message passing it is not a requirement that sender nor receiver use object-oriented programming. Procedural language systems can be wrapped and treated as large grained objects capable of sending and receiving messages.[4] Examples of systems that support distributed objects are:Emerald,ONC RPC,CORBA,Java RMI,DCOM,SOAP,.NET Remoting,CTOS,QNX Neutrino RTOS,OpenBinderandD-Bus. Distributed object systems have been called "shared nothing" systems because the message passing abstraction hides underlying state changes that may be used in the implementation of sending messages. Distributed, or asynchronous, message-passing has additional overhead compared to calling a procedure. In message-passing, arguments must be copied to the new message. Some arguments can contain megabytes of data, all of which must be copied and transmitted to the receiving object. Traditionalprocedure callsdiffer from message-passing in terms of memory usage,transfer timeand locality. Arguments are passed to the receiver typically bygeneral-purpose registersrequiring no additional storage nor transfer time, or in aparameterlist containing the arguments' addresses (a few bits). Address-passing is not possible for distributed systems since the systems use separate address spaces. Webbrowsersandweb serversare examples of processes that communicate by message-passing. AURLis an example of referencing a resource without exposing process internals. Asubroutinecall ormethodinvocation will not exit until the invoked computation has terminated. Asynchronous message-passing, by contrast, can result in a response arriving a significant time after the request message was sent. A message-handler will, in general, process messages from more than one sender. This means its state can change for reasons unrelated to the behavior of a single sender or client process. This is in contrast to the typical behavior of an object upon which methods are being invoked: the latter is expected to remain in the same state between method invocations. In other words, the message-handler behaves analogously to avolatile object. The prominent mathematical models of message passing are theActor modelandPi calculus.[5][6]In mathematical terms a message is the single means to pass control to an object. If the object responds to the message, it has amethodfor that message. Alan Kayhas argued that message passing is more important than objects in OOP, and that objects themselves are often over-emphasized. Thelive distributed objectsprogramming model builds upon this observation; it uses the concept of adistributed data flowto characterize the behavior of a complex distributed system in terms of message patterns, using high-level, functional-style specifications.[7]	https://en.wikipedia.org/wiki/Message_Passing
Insoftware designandengineering, theobserver patternis asoftware design patternin which anobject, named thesubject, maintains a list of its dependents, calledobservers, and notifies them automatically of anystate changes, usually by calling one of theirmethods. It is often used for implementing distributedevent-handlingsystems inevent-driven software. In such systems, the subject is usually named a "stream of events" or "stream source of events" while the observers are called "sinks of events." The stream nomenclature alludes to a physical setup in which the observers are physically separated and have no control over the emitted events from the subject/stream source. This pattern thus suits any process by which data arrives from some input that is not available to theCPUatstartup, but instead may arrive at arbitrary or indeterminate times (HTTP requests,GPIOdata,user inputfromperipheralsanddistributed databases, etc.). The observer design pattern is a behavioural pattern listed among the 23 well-known"Gang of Four" design patternsthat address recurring design challenges in order to design flexible and reusable object-oriented software, yielding objects that are easier to implement, change, test and reuse.[1] The observer pattern addresses the following problems:[2] Defining a one-to-many dependency between objects by defining one object (subject) that updates the state of dependent objects directly is inflexible because it couples the subject to particular dependent objects. However, it might be applicable from a performance point of view or if the object implementation is tightly coupled (such as low-level kernel structures that execute thousands of times per second). Tightly coupled objects can be difficult to implement in some scenarios and are not easily reused because they refer to and are aware of many objects with different interfaces. In other scenarios, tightly coupled objects can be a better option because the compiler is able to detect errors at compile time and optimize the code at the CPU instruction level. The sole responsibility of a subject is to maintain a list of observers and to notify them of state changes by calling theirupdate()operation. The responsibility of observers is to register and unregister themselves with a subject (in order to be notified of state changes) and to update their state (to synchronize their state with the subject's state) when they are notified. This makes subject and observers loosely coupled. Subject and observers have no explicit knowledge of each other. Observers can be added and removed independently at run time. This notification-registration interaction is also known aspublish-subscribe. The observer pattern can causememory leaks, known as thelapsed listener problem, because in a basic implementation, it requires both explicit registration and explicit deregistration, as in thedispose pattern, because the subject holds strong references to the observers, keeping them alive. This can be prevented if the subject holdsweak referencesto the observers. Typically, the observer pattern is implemented so that the subject being observed is part of the object for which state changes are being observed (and communicated to the observers). This type of implementation is consideredtightly coupled, forcing both the observers and the subject to be aware of each other and have access to their internal parts, creating possible issues ofscalability, speed, message recovery and maintenance (also called event or notification loss), the lack of flexibility in conditional dispersion and possible hindrance to desired security measures. In some (non-polling) implementations of thepublish-subscribe pattern, this is solved by creating a dedicated message queue server (and sometimes an extra message handler object) as an extra stage between the observer and the object being observed, thus decoupling the components. In these cases, the message queue server is accessed by the observers with the observer pattern, subscribing to certain messages and knowing (or not knowing, in some cases) about only the expected message, while knowing nothing about the message sender itself; the sender may also know nothing about the observers. Other implementations of the publish-subscribe pattern, which achieve a similar effect of notification and communication to interested parties, do not use the observer pattern.[3][4] In early implementations of multi-window operating systems such asOS/2andWindows, the terms "publish-subscribe pattern" and "event-driven software development" were used as synonyms for the observer pattern.[5] The observer pattern, as described in theDesign Patternsbook, is a very basic concept and does not address removing interest in changes to the observed subject or special logic to be performed by the observed subject before or after notifying the observers. The pattern also does not deal with recording change notifications or guaranteeing that they are received. These concerns are typically handled in message-queueing systems, in which the observer pattern plays only a small part. Related patterns include publish–subscribe,mediatorandsingleton. The observer pattern may be used in the absence of publish-subscribe, as when model status is frequently updated. Frequent updates may cause the view to become unresponsive (e.g., by invoking manyrepaintcalls); such observers should instead use a timer. Instead of becoming overloaded by change message, the observer will cause the view to represent the approximate state of the model at a regular interval. This mode of observer is particularly useful forprogress bars, in which the underlying operation's progress changes frequently. In thisUMLclass diagram, theSubjectclass does not update the state of dependent objects directly. Instead,Subjectrefers to theObserverinterface (update()) for updating state, which makes theSubjectindependent of how the state of dependent objects is updated. TheObserver1andObserver2classes implement theObserverinterface by synchronizing their state with subject's state. TheUMLsequence diagramshows the runtime interactions: TheObserver1andObserver2objects callattach(this)onSubject1to register themselves. Assuming that the state ofSubject1changes,Subject1callsnotify()on itself.notify()callsupdate()on the registeredObserver1andObserver2objects, which request the changed data (getState()) fromSubject1to update (synchronize) their state. While the library classesjava.util.Observerandjava.util.Observableexist, they have beendeprecatedin Java 9 because the model implemented was quite limited. Below is an example written inJavathat takes keyboard input and handles each input line as an event. When a string is supplied fromSystem.in, the methodnotifyObservers()is then called in order to notify all observers of the event's occurrence, in the form of an invocation of their update methods. This is a C++11 implementation. The program output is like Output A similar example inPython: C# provides theIObservable.[7]andIObserver[8]interfaces as well as documentation on how to implement the design pattern.[9] JavaScript has a deprecatedObject.observefunction that was a more accurate implementation of the observer pattern.[10]This would fire events upon change to the observed object. Without the deprecatedObject.observefunction, the pattern may be implemented with more explicit code:[11]	https://en.wikipedia.org/wiki/Observer_pattern
Insoftware architecture,publish–subscribeorpub/subis amessaging patternwhere publishers categorizemessagesinto classes that are received by subscribers. This is contrasted to the typical messaging pattern model where publishers send messages directly to subscribers. Similarly, subscribers express interest in one or more classes and only receive messages that are of interest, without knowledge of which publishers, if any, there are. Publish–subscribe is a sibling of themessage queueparadigm, and is typically one part of a largermessage-oriented middlewaresystem. Most messaging systems support both the pub/sub and message queue models in theirAPI; e.g.,Java Message Service(JMS). This pattern provides greater networkscalabilityand a more dynamicnetwork topology, with a resulting decreased flexibility to modify the publisher and the structure of the published data. According to Gregor Hohpe, compared with synchronous messaging patterns (such asRPC) andpoint-to-point messagingpatterns, publish–subscribe provides the highest level ofdecouplingamong architectural components, however it can alsocouplethem in some other ways (such as format and semantic coupling) so they become messy over time.[1] In the publish–subscribe model, subscribers typically receive only a subset of the total messages published. The process of selecting messages for reception and processing is calledfiltering. There are two common forms of filtering: topic-based and content-based. In atopic-basedsystem, messages are published to "topics" or named logical channels. Subscribers in a topic-based system will receive all messages published to the topics to which they subscribe. The publisher is responsible for defining the topics to which subscribers can subscribe. In acontent-basedsystem, messages are only delivered to a subscriber if the attributes or content of those messages matches constraints defined by the subscriber. The subscriber is responsible for classifying the messages. Some systems support ahybridof the two; publishers post messages to a topic while subscribers register content-based subscriptions to one or more topics. In many publish–subscribe systems, publishers post messages to an intermediarymessage broker or event bus, and subscribers register subscriptions with that broker, letting the broker perform the filtering. The broker normally performs astore and forwardfunction to route messages from publishers to subscribers. In addition, the broker may prioritize messages in aqueuebefore routing.[citation needed] Subscribers may register for specific messages at build time, initialization time or runtime. In GUI systems, subscribers can be coded to handle user commands (e.g., click of a button), which corresponds to build time registration. Some frameworks and software products useXMLconfiguration files to register subscribers. These configuration files are read at initialization time. The most sophisticated alternative is when subscribers can be added or removed at runtime. This latter approach is used, for example, indatabase triggers,mailing lists, andRSS.[citation needed] TheData Distribution Service(DDS) middleware does not use a broker in the middle. Instead, each publisher and subscriber in the pub/sub system shares meta-data about each other viaIP multicast. The publisher and the subscribers cache this information locally and route messages based on the discovery of each other in the shared cognizance. In effect, brokerless architectures require publish/subscribe system to construct an overlay network which allows efficient decentralized routing from publishers to subscribers. It was shown byJon Kleinbergthat efficient decentralised routing requiresNavigable Small-World topologies. Such Small-World topologies are usually implemented by decentralized or federated publish/subscribe systems.[2]Locality-aware publish/subscribe systems[3]construct Small-World topologies that route subscriptions through short-distance and low-cost links thereby reducing subscription delivery times. One of the earliest publicly described pub/sub systems was the "news" subsystem of the Isis Toolkit, described at the 1987Association for Computing Machinery(ACM) Symposium on Operating Systems Principles conference (SOSP '87), in a paper "ExploitingVirtual SynchronyinDistributed Systems. 123–138."[4] Publishers areloosely coupledto subscribers, and need not even know of their existence. With the topic being the focus, publishers and subscribers are allowed to remain ignorant of system topology. Each can continue to operate as per normal independently of the other. In the traditional tightly coupledclient–server paradigm, the client cannot post messages to the server while the server process is not running, nor can the server receive messages unless the client is running. Many pub/sub systems decouple not only the locations of the publishers and subscribers but also decouple them temporally. A common strategy used bymiddleware analystswith such pub/sub systems is to take down a publisher to allow the subscriber to work through the backlog (a form ofbandwidth throttling). Pub/sub provides the opportunity for betterscalabilitythan traditional client-server, through parallel operation, message caching, tree-based or network-based routing, etc. However, in certain types of tightly coupled, high-volume enterprise environments, as systems scale up to become data centers with thousands of servers sharing the pub/sub infrastructure, current vendor systems often lose this benefit; scalability for pub/sub products under high load in these contexts is a research challenge. Outside of the enterprise environment, on the other hand, the pub/sub paradigm has proven its scalability to volumes far beyond those of a single data center, providing Internet-wide distributed messaging through web syndication protocols such asRSSandAtom. These syndication protocols accept higher latency and lack of delivery guarantees in exchange for the ability for even a low-end web server to syndicate messages to (potentially) millions of separate subscriber nodes. The most serious problems with pub/sub systems are a side-effect of their main advantage: the decoupling of publisher from subscriber. A pub/sub system must be designed carefully to be able to provide stronger system properties that a particular application might require, such as assured delivery. The pub/sub pattern scales well for small networks with a small number of publisher and subscriber nodes and low message volume. However, as the number of nodes and messages grows, the likelihood of instabilities increases, limiting the maximum scalability of a pub/sub network. Example throughput instabilities at large scales include: For pub/sub systems that use brokers (servers), the argument for a broker to send messages to a subscriber isin-band, and can be subject to security problems. Brokers might be fooled into sending notifications to the wrong client, amplifying denial of service requests against the client. Brokers themselves could be overloaded as they allocate resources to track created subscriptions. Even with systems that do not rely on brokers, a subscriber might be able to receive data that it is not authorized to receive. An unauthorized publisher may be able to introduce incorrect or damaging messages into the pub/sub system. This is especially true with systems thatbroadcastormulticasttheir messages.Encryption(e.g.Transport Layer Security(SSL/TLS)) can prevent unauthorized access, but cannot prevent damaging messages from being introduced by authorized publishers. Architectures other than pub/sub, such as client/server systems, are also vulnerable to authorized message senders that behave maliciously.	https://en.wikipedia.org/wiki/Publish/subscribe
Theservice locator patternis adesign patternused insoftware developmentto encapsulate the processes involved in obtaining a service with a strongabstraction layer. This pattern uses a central registry known as the "service locator", which on request returns the information necessary to perform a certain task.[1]Proponents of the pattern say the approach simplifies component-based applications where all dependencies are cleanly listed at the beginning of the whole application design, consequently making traditional dependency injection a more complex way of connecting objects. Critics of the pattern argue that it is ananti-patternwhich obscures dependencies and makes software harder to test.[2][better source needed] Thissoftware-engineering-related article is astub. You can help Wikipedia byexpanding it.	https://en.wikipedia.org/wiki/Service_locator_pattern
Signalsare standardized messages sent to a runningprogramto trigger specific behavior, such as quitting or error handling. They are a limited form ofinter-process communication(IPC), typically used inUnix,Unix-like, and otherPOSIX-compliant operating systems. A signal is anasynchronousnotification sent to aprocessor to a specificthreadwithin the same process to notify it of an event. Common uses of signals are to interrupt, suspend, terminate orkilla process. Signals originated in 1970sBell LabsUnix and were later specified in thePOSIXstandard. When a signal is sent, the operating system interrupts the target process's normalflow of executionto deliver the signal. Execution can be interrupted during anynon-atomic instruction. If the process has previously registered asignal handler, that routine is executed. Otherwise, the default signal handler is executed. Embedded programs may find signals useful for inter-process communications, as signals are notable for theiralgorithmic efficiency. Signals are similar tointerrupts, the difference being that interrupts are mediated by theCPUand handled by thekernelwhile signals are mediated by the kernel (possibly via system calls) and handled by individualprocesses.[citation needed]The kernel may pass an interrupt as a signal to the process that caused it (typical examples areSIGSEGV,SIGBUS,SIGILLandSIGFPE). Thekill(2)system call sends a specified signal to a specified process, if permissions allow. Similarly, thekill(1)command allows a user to send signals to processes. Theraise(3)library function sends the specified signal to the current process. Exceptionssuch asdivision by zero,segmentation violation(SIGSEGV), and floating point exception (SIGFPE) will cause acore dumpand terminate the program. The kernel can generate signals to notify processes of events. For example,SIGPIPEwill be generated when a process writes to a pipe which has been closed by the reader; by default, this causes the process to terminate, which is convenient when constructingshell pipelines. Typing certain key combinations at thecontrolling terminalof a running process causes the system to send it certain signals:[3] These default key combinations with modern operating systems can be changed with thesttycommand. Signal handlers can be installed with thesignal(2)orsigaction(2)system call. If a signal handler is not installed for a particular signal, the default handler is used. Otherwise the signal is intercepted and the signal handler is invoked. The process can also specify two default behaviors, without creating a handler: ignore the signal (SIG_IGN) and use the default signal handler (SIG_DFL). There are two signals which cannot be intercepted and handled:SIGKILLandSIGSTOP. Signal handling is vulnerable torace conditions. As signals are asynchronous, another signal (even of the same type) can be delivered to the process during execution of the signal handling routine. Thesigprocmask(2)call can be used to block and unblock delivery of signals. Blocked signals are not delivered to the process until unblocked. Signals that cannot be ignored (SIGKILL and SIGSTOP) cannot be blocked. Signals can cause the interruption of a system call in progress, leaving it to the application to manage anon-transparent restart. Signal handlers should be written in a way that does not result in any unwanted side-effects, e.g.errnoalteration, signal mask alteration, signal disposition change, and other globalprocessattribute changes. Use of non-reentrantfunctions, e.g.,mallocorprintf, inside signal handlers is also unsafe. In particular, the POSIX specification and the Linux man pagesignal (7)require that all system functions directly orindirectlycalled from a signal function areasync-signal safe.[6][7]Thesignal-safety(7)man page gives a list of such async-signal safe system functions (practically thesystem calls), otherwise it is anundefined behavior.[8]It is suggested to simply set somevolatile sig_atomic_tvariable in a signal handler, and to test it elsewhere.[9] Signal handlers can instead put the signal into aqueueand immediately return. The main thread will then continue "uninterrupted" until signals are taken from the queue, such as in anevent loop. "Uninterrupted" here means that operations thatblockmay return prematurely andmust be resumed, as mentioned above. Signals should be processed from the queue on the main thread and not byworker pools, as that reintroduces the problem of asynchronicity. However, managing a queue is not possible in an async-signal safe way with onlysig_atomic_t, as only single reads and writes to such variables are guaranteed to be atomic, not increments or (fetch-and)-decrements, as would be required for a queue. Thus, effectively, only one signal per handler can be queued safely withsig_atomic_tuntil it has been processed. Aprocess's execution may result in the generation of a hardwareexception, for instance, if the process attempts to divide by zero or incurs apage fault. InUnix-likeoperating systems, this event automatically changes the processorcontextto start executing akernelexception handler. In case of some exceptions, such as apage fault, the kernel has sufficient information to fully handle the event itself and resume the process's execution. Other exceptions, however, the kernel cannot process intelligently and it must instead defer the exception handling operation to the faulting process. This deferral is achieved via the signal mechanism, wherein the kernel sends to the process a signal corresponding to the current exception. For example, if a process attempted integer divide by zero on anx86CPU, adivide errorexception would be generated and cause the kernel to send theSIGFPEsignal to the process. Similarly, if the process attempted to access a memory address outside of itsvirtual address space, the kernel would notify the process of this violation via aSIGSEGV(segmentation violationsignal). The exact mapping between signal names and exceptions is obviously dependent upon the CPU, since exception types differ between architectures. The list below documents the signals specified in theSingle Unix SpecificationVersion 5. All signals are defined as macro constants in the<signal.h>header file. The name of the macro constant consists of a "SIG"prefixfollowed by a mnemonic name for the signal. A process can definehow to handle incoming POSIX signals. If a process does not define a behaviour for a signal, then thedefault handlerfor that signal is being used. The table below lists some default actions for POSIX-compliant UNIX systems, such asFreeBSD,OpenBSDandLinux. The following signals are not specified in thePOSIXspecification. They are, however, sometimes used on various systems.	https://en.wikipedia.org/wiki/Signal_(computing)
Incomputer programming, asoftware frameworkis asoftwareabstractionthat provides generic functionality which developers can extend with custom code to create applications. It establishes a standard foundation for building and deploying software, offering reusable components and design patterns that handle common programming tasks within a largersoftware platformor environment. Unlikelibrarieswhere developers call functions as needed, frameworks implement inversion of control by dictating program structure and calling user code at specific points, while also providing default behaviors, structured extensibility mechanisms, and maintaining a fixed core that accepts extensions without direct modification. Frameworks also differ from regular applications that can be modified (likeweb browsersthroughextensions, video games throughmods), in that frameworks are intentionally incomplete scaffolding meant to be extended through well-defined extension points and following specific architectural patterns. Software frameworks may include support programs, compilers, code libraries, toolsets, andAPIsthat bring together all the differentcomponentsto enable development of aprojectorsystem. Software frameworks differ from standardlibrariesin key ways: The designers of software frameworks aim to facilitate software developments by allowing designers and programmers to devote their time to meeting software requirements rather than dealing with the more standard low-level details of providing a working system, thereby reducing overall development time.[1]For example, a team using aweb frameworkto develop a banking website can focus on writing code particular to banking rather than the mechanics of request handling andstate management. Frameworks often add to the size of programs, a phenomenon termed "code bloat". Due to customer-demand-driven applications needs, both competing and complementary frameworks sometimes end up in a product. Further, due to the complexity of their APIs, the intended reduction in overall development time may not be achieved due to the need to spend additional time learning to use the framework; this criticism is clearly valid when a special or new framework is first encountered by development staff.[citation needed]If such a framework is not used in subsequent job taskings, the time invested in learning the framework can cost more than purpose-written code familiar to the project's staff; many programmers keep copies of usefulboilerplate codefor common needs. However, once a framework is learned, future projects can be faster and easier to complete; the concept of a framework is to make a one-size-fits-all solution set, and with familiarity, code production should logically rise. There are no such claims made about the size of the code eventually bundled with the output product, nor its relative efficiency and conciseness. Using any library solution necessarily pulls in extras and unused extraneous assets unless the software is a compiler-object linker making a tight (small, wholly controlled, and specified) executable module. The issue continues, but a decade-plus of industry experience[citation needed]has shown that the most effective frameworks turn out to be those that evolve fromre-factoringthe common code of the enterprise, instead of using a generic "one-size-fits-all" framework developed by third parties for general purposes. An example of that would be how the user interface in such an application package as an office suite grows to have common look, feel, and data-sharing attributes and methods, as the once disparate bundled applications, grow unified into a suite that is tighter and smaller; the newer/evolved suite can be a product that shares integral utility libraries and user interfaces. This trend in the controversy brings up an important issue about frameworks. Creating a framework that is elegant, versus one that merely solves a problem, is still rather a craft than a science. "Softwareelegance" implies clarity, conciseness, and little waste (extra or extraneous functionality, much of which is user-defined). For those frameworks that generate code, for example, "elegance" would imply the creation of code that is clean and comprehensible to a reasonably knowledgeable programmer (and which is therefore readily modifiable), versus one that merely generates correct code. The elegance issue is why relatively few software frameworks have stood the test of time: the best frameworks have been able to evolve gracefully as the underlying technology on which they were built advanced. Even there, having evolved, many such packages will retain legacy capabilities bloating the final software as otherwise replaced methods have been retained in parallel with the newer methods. Software frameworks typically contain considerable housekeeping and utility code in order to help bootstrap user applications, but generally focus on specific problem domains, such as: According to Pree,[7]software frameworks consist offrozen spotsandhot spots.Frozen spotsdefine the overall architecture of a software system, that is to say its basic components and the relationships between them. These remain unchanged (frozen) in any instantiation of the application framework.Hot spotsrepresent those parts where the programmers using the framework add their own code to add the functionality specific to their own project. In anobject-orientedenvironment, a framework consists ofabstractandconcreteclasses.Instantiationof such a framework consists ofcomposingandsubclassingthe existing classes.[8] The necessary functionality can be implemented by using theTemplate Method Patternin which thefrozen spotsare known as invariant methods and thehot spotsare known as variant or hook methods. The invariant methods in the superclass provide default behaviour while the hook methods in each subclass provide custom behaviour. When developing a concrete software system with a software framework, developers utilize the hot spots according to the specific needs and requirements of the system. Software frameworks rely on theHollywood Principle: "Don't call us, we'll call you."[9][10]This means that the user-defined classes (for example, new subclasses) receive messages from the predefined framework classes. Developers usually handle this by implementingsuperclassabstract methods.	https://en.wikipedia.org/wiki/Software_framework
Incomputer programming, thestrategy pattern(also known as thepolicy pattern) is abehavioralsoftware design patternthat enables selecting analgorithmat runtime. Instead of implementing a single algorithm directly, code receives runtime instructions as to which in a family of algorithms to use.[1] Strategy lets the algorithm vary independently from clients that use it.[2]Strategy is one of the patterns included in the influential bookDesign Patternsby Gamma et al.[3]that popularized the concept of using design patterns to describe how to design flexible and reusable object-oriented software. Deferring the decision about which algorithm to use until runtime allows the calling code to be more flexible and reusable. For instance, a class that performs validation on incoming data may use the strategy pattern to select a validation algorithm depending on the type of data, the source of the data, user choice, or other discriminating factors. These factors are not known until runtime and may require radically different validation to be performed. The validation algorithms (strategies), encapsulated separately from the validating object, may be used by other validating objects in different areas of the system (or even different systems) withoutcode duplication. Typically, the strategy pattern stores a reference to code in a data structure and retrieves it. This can be achieved by mechanisms such as the nativefunction pointer, thefirst-class function, classes or class instances inobject-oriented programminglanguages, or accessing the language implementation's internal storage of code viareflection. In the aboveUMLclass diagram, theContextclass does not implement an algorithm directly. Instead,Contextrefers to theStrategyinterface for performing an algorithm (strategy.algorithm()), which makesContextindependent of how an algorithm is implemented. TheStrategy1andStrategy2classes implement theStrategyinterface, that is, implement (encapsulate) an algorithm.TheUMLsequence diagramshows the runtime interactions: TheContextobject delegates an algorithm to differentStrategyobjects. First,Contextcallsalgorithm()on aStrategy1object, which performs the algorithm and returns the result toContext. Thereafter,Contextchanges its strategy and callsalgorithm()on aStrategy2object, which performs the algorithm and returns the result toContext. [5] According to the strategy pattern, the behaviors of a class should not be inherited. Instead, they should be encapsulated using interfaces. This is compatible with theopen/closed principle(OCP), which proposes that classes should be open for extension but closed for modification. As an example, consider a car class. Two possible functionalities for car arebrakeandaccelerate. Since accelerate and brake behaviors change frequently between models, a common approach is to implement these behaviors in subclasses. This approach has significant drawbacks; accelerate and brake behaviors must be declared in each new car model. The work of managing these behaviors increases greatly as the number of models increases, and requires code to be duplicated across models. Additionally, it is not easy to determine the exact nature of the behavior for each model without investigating the code in each. The strategy pattern usescomposition instead of inheritance. In the strategy pattern, behaviors are defined as separate interfaces and specific classes that implement these interfaces. This allows better decoupling between the behavior and the class that uses the behavior. The behavior can be changed without breaking the classes that use it, and the classes can switch between behaviors by changing the specific implementation used without requiring any significant code changes. Behaviors can also be changed at runtime as well as at design-time. For instance, a car object's brake behavior can be changed fromBrakeWithABS()toBrake()by changing thebrakeBehaviormember to:	https://en.wikipedia.org/wiki/Strategy_pattern
Auser exitis asubroutineinvoked by asoftwarepackage for a predefined event in the execution of the package. In some cases the exit is specified by the installation when configuring the package while in other cases the users of the package can substitute their own subroutines in place of the default ones provided by the package vendor to provide customized functionality. In some cases security controls restrict exits to authorized users, e.g.,EXCPappendagesinMVS. The earliest use of this term involved operating systems to let customized code temporarily take control when a pre-designated event occurred.[1][2] A more typical use is replacing the user exits provided by asort/mergepackage,[3][4][5]whereby the user program provides its own subroutines for comparing records. The procedures provided by the user take the place of the default routines (usually stubs that do nothing butreturnto their caller) provided by the package vendor. Procedures provided as user exits are typicallycompiledinto astatic libraryandlinkeddirectly with the package to produce anexecutable program. Another approach employsdynamic librariesto accomplish the same thing. Finally, user exits may be external programs executed at specific points in the execution of a host package.[6] Historically, this term is commonly used in IBM mainframe vernacular.[1][2] If the user site specific code is substituted for the software vendor provided default exit it must interface to the software package using the defined parameters as documented for the default exit. User exits are important because while they enable site specific customization they isolate such installation specific customization to defined and supported points enabling the site to upgrade to follow-on releases of the software package without adverse impact to preexisting customized functionality. Some references to IBM user exit manuals are given below. Other vendors such as SAP, Oracle, IFS, HP, Macro4, Compuware, CA all employ user exits in some of their software products. In SAP, A 'COMMIT WORK' must never be used inside a User Exit, as it may affect the program processing. Furthermore, error messages may not be issued from inside an exit, as they halt the processing of the code that follows the message. If files containing user-created content are meant to follow a standard, the (file)Opencommand of a word-processing document can invoke a user exit that does validation and, if necessary, returns an error code.[7] Some applications that provide user exits:	https://en.wikipedia.org/wiki/User_exit
Avisitor patternis asoftware design patternthat separates thealgorithmfrom theobjectstructure. Because of this separation, new operations can be added to existing object structures without modifying the structures. It is one way to follow theopen/closed principleinobject-oriented programmingandsoftware engineering. In essence, the visitor allows adding newvirtual functionsto a family ofclasses, without modifying the classes. Instead, a visitor class is created that implements all of the appropriate specializations of the virtual function. The visitor takes the instance reference as input, and implements the goal throughdouble dispatch. Programming languages withsum typesandpattern matchingobviate many of the benefits of the visitor pattern, as the visitor class is able to both easily branch on the type of the object and generate a compiler error if a new object type is defined which the visitor does not yet handle. The Visitor[1]design pattern is one of the twenty-three well-knownGang of Four design patternsthat describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change, test, and reuse. When new operations are needed frequently and the object structure consists of many unrelated classes, it's inflexible to add new subclasses each time a new operation is required because "[..] distributing all these operations across the various node classes leads to a system that's hard to understand, maintain, and change."[1] This makes it possible to create new operations independently from the classes of an object structure by adding new visitor objects. See also the UML class and sequence diagram below. TheGang of Fourdefines the Visitor as: Represent[ing] an operation to be performed on elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates. The nature of the Visitor makes it an ideal pattern to plug into public APIs, thus allowing its clients to perform operations on a class using a "visiting" class without having to modify the source.[2] Moving operations into visitor classes is beneficial when A drawback to this pattern, however, is that it makes extensions to the class hierarchy more difficult, as new classes typically require a newvisitmethod to be added to each visitor. Consider the design of a 2Dcomputer-aided design(CAD) system. At its core, there are several types to represent basic geometric shapes like circles, lines, and arcs. The entities are ordered into layers, and at the top of the type hierarchy is the drawing, which is simply a list of layers, plus some added properties. A fundamental operation on this type hierarchy is saving a drawing to the system's native file format. At first glance, it may seem acceptable to add local save methods to all types in the hierarchy. But it is also useful to be able to save drawings to other file formats. Adding ever more methods for saving into many different file formats soon clutters the relatively pure original geometric data structure. A naive way to solve this would be to maintain separate functions for each file format. Such a save function would take a drawing as input, traverse it, and encode into that specific file format. As this is done for each added different format, duplication between the functions accumulates. For example, saving a circle shape in a raster format requires very similar code no matter what specific raster form is used, and is different from other primitive shapes. The case for other primitive shapes like lines and polygons is similar. Thus, the code becomes a large outer loop traversing through the objects, with a large decision tree inside the loop querying the type of the object. Another problem with this approach is that it is very easy to miss a shape in one or more savers, or a new primitive shape is introduced, but the save routine is implemented only for one file type and not others, leading to code extension and maintenance problems. As the versions of the same file grows it becomes more complicated to maintain it. Instead, the visitor pattern can be applied. It encodes the logical operation (i.e. save(image_tree)) on the whole hierarchy into one class (i.e. Saver) that implements the common methods for traversing the tree and describes virtual helper methods (i.e. save_circle, save_square, etc.) to be implemented for format specific behaviors. In the case of the CAD example, such format specific behaviors would be implemented by a subclass of Visitor (i.e. SaverPNG). As such, all duplication of type checks and traversal steps is removed. Additionally, the compiler now complains if a shape is omitted since it is now expected by the common base traversal/save function. The visitor pattern may be used for iteration overcontainer-like data structures just likeIterator patternbut with limited functionality.[3]: 288For example,iterationover a directory structure could be implemented by a function class instead of more conventionalloop pattern. This would allow deriving various useful information from directories content by implementing a visitor functionality for every item whilereusingthe iteration code. It's widely employed in Smalltalk systems and can be found in C++ as well.[3]: 289A drawback of this approach, however, is that you can't break out of the loop easily or iterate concurrently (in parallel i.e. traversing two containers at the same time by a singleivariable).[3]: 289The latter would require writing additional functionality for a visitor to support these features.[3]: 289 In theUMLclass diagramabove, theElementAclass doesn't implement a new operation directly. Instead,ElementAimplements adispatching operationaccept(visitor)that "dispatches" (delegates) a request to the "accepted visitor object" (visitor.visitElementA(this)). TheVisitor1class implements the operation (visitElementA(e:ElementA)).ElementBthen implementsaccept(visitor)by dispatching tovisitor.visitElementB(this). TheVisitor1class implements the operation (visitElementB(e:ElementB)). TheUMLsequence diagramshows the run-time interactions: TheClientobject traverses the elements of an object structure (ElementA,ElementB) and callsaccept(visitor)on each element.First, theClientcallsaccept(visitor)onElementA, which callsvisitElementA(this)on the acceptedvisitorobject. The element itself (this) is passed to thevisitorso that it can "visit"ElementA(calloperationA()).Thereafter, theClientcallsaccept(visitor)onElementB, which callsvisitElementB(this)on thevisitorthat "visits"ElementB(callsoperationB()). The visitor pattern requires aprogramming languagethat supportssingle dispatch, as common object-oriented languages (such asC++,Java,Smalltalk,Objective-C,Swift,JavaScript,PythonandC#) do. Under this condition, consider two objects, each of some class type; one is termed theelement, and the other isvisitor. Thevisitordeclares avisitmethod, which takes the element as an argument, for each class of element.Concrete visitorsare derived from the visitor class and implement thesevisitmethods, each of which implements part of the algorithm operating on the object structure. The state of the algorithm is maintained locally by the concrete visitor class. Theelementdeclares anacceptmethod to accept a visitor, taking the visitor as an argument.Concrete elements, derived from the element class, implement theacceptmethod. In its simplest form, this is no more than a call to the visitor'svisitmethod.Compositeelements, which maintain a list of child objects, typically iterate over these, calling each child'sacceptmethod. Theclientcreates the object structure, directly or indirectly, and instantiates the concrete visitors. When an operation is to be performed which is implemented using the Visitor pattern, it calls theacceptmethod of the top-level element(s). When theacceptmethod is called in the program, its implementation is chosen based on both the dynamic type of the element and the static type of the visitor. When the associatedvisitmethod is called, its implementation is chosen based on both the dynamic type of the visitor and the static type of the element, as known from within the implementation of theacceptmethod, which is the same as the dynamic type of the element. (As a bonus, if the visitor can't handle an argument of the given element's type, then the compiler will catch the error.) Thus, the implementation of thevisitmethod is chosen based on both the dynamic type of the element and the dynamic type of the visitor. This effectively implementsdouble dispatch. For languages whose object systems support multiple dispatch, not only single dispatch, such asCommon LisporC#via theDynamic Language Runtime(DLR), implementation of the visitor pattern is greatly simplified (a.k.a. Dynamic Visitor) by allowing use of simple function overloading to cover all the cases being visited. A dynamic visitor, provided it operates on public data only, conforms to theopen/closed principle(since it does not modify extant structures) and to thesingle responsibility principle(since it implements the Visitor pattern in a separate component). In this way, one algorithm can be written to traverse a graph of elements, and many different kinds of operations can be performed during that traversal by supplying different kinds of visitors to interact with the elements based on the dynamic types of both the elements and the visitors. This example declares a separateExpressionPrintingVisitorclass that takes care of the printing. If the introduction of a new concrete visitor is desired, a new class will be created to implement the Visitor interface, and new implementations for the Visit methods will be provided. The existing classes (Literal and Addition) will remain unchanged. In this case, it is the object's responsibility to know how to print itself on a stream. The visitor here is then the object, not the stream. Go does not support method overloading, so the visit methods need different names. A typical visitor interface might be The following example is in the languageJava, and shows how the contents of a tree of nodes (in this case describing the components of a car) can be printed. Instead of creatingprintmethods for each node subclass (Wheel,Engine,Body, andCar), one visitor class (CarElementPrintVisitor) performs the required printing action. Because different node subclasses require slightly different actions to print properly,CarElementPrintVisitordispatches actions based on the class of the argument passed to itsvisitmethod.CarElementDoVisitor, which is analogous to a save operation for a different file format, does likewise. Theother-objectparameter is superfluous intraverse. The reason is that it is possible to use an anonymous function that calls the desired target method with a lexically captured object: Now, the multiple dispatch occurs in the call issued from the body of the anonymous function, and sotraverseis just a mapping function that distributes a function application over the elements of an object. Thus all traces of the Visitor Pattern disappear, except for the mapping function, in which there is no evidence of two objects being involved. All knowledge of there being two objects and a dispatch on their types is in the lambda function. Python does not support method overloading in the classical sense (polymorphic behavior according to type of passed parameters), so the "visit" methods for the different model types need to have different names. Using Python 3 or above allows to make a general implementation of the accept method: One could extend this to iterate over the class's method resolution order if they would like to fall back on already-implemented classes. They could also use the subclass hook feature to define the lookup in advance.	https://en.wikipedia.org/wiki/Visitor_pattern
XSLT(Extensible Stylesheet Language Transformations) is a language originally designed fortransformingXMLdocuments into other XML documents,[1]or other formats such asHTMLforweb pages,plain text, orXSL Formatting Objects. These formats can be subsequently converted to formats such asPDF,PostScript, andPNG.[2]Support for JSON and plain-text transformation was added in later updates to the XSLT 1.0 specification. XSLT 3.0 implementations support Java, .NET, C/C++, Python, PHP and NodeJS. An XSLT 3.0 JavaScript library can also be hosted within the web browser. Modern web browsers also include native support for XSLT 1.0.[3] The XSLT document transformation specifies how to transform an XML document into new document (usually XML, but other formats, such as plain text are supported).[4]Typically, input documents are XML files, but anything from which the processor can build anXQuery and XPath Data Modelcan be used, such asrelational databasetables orgeographical information systems.[1] While XSLT was originally designed as a special-purpose language for XML transformation, the language isTuring-complete, making it theoretically capable of arbitrary computations.[5] XSLT is influenced byfunctional languages,[6]and by text-based pattern matching languages likeSNOBOLandAWK. Its most direct predecessor isDSSSL, which did forSGMLwhat XSLT does for XML.[7] The XSLT processor takes one or more XML source documents, plus one or more XSLT stylesheets, and processes them to produce one or multiple output documents.[16][17]In contrast to widely implementedimperative programminglanguages likeC, XSLT isdeclarative.[18]The basic processing paradigm is pattern matching.[19]Rather than listing an imperative sequence of actions to perform in a stateful environment, template rules only define how to handle a node matching a particular XPath-like pattern, if the processor should happen to encounter one, and the contents of the templates effectively comprisefunctionalexpressionsthat directly represent their evaluated form: the result tree, which is the basis of the processor's output. A typical processor behaves as follows. First, assuming a stylesheet has already been read and prepared, the processor builds a sourcetreefrom the input XML document. It then processes the source tree's root node, finds the best-matching template for that node in the stylesheet, and evaluates the template's contents. Instructions in each template generally direct the processor to either create nodes in the result tree, or to process more nodes in the source tree in the same way as the root node. Finally the result tree is serialized as XML or HTML text. XSLT usesXPathto identify subsets of the source document tree and perform calculations. XPath also provides a range offunctions, which XSLT itself further augments. XSLT 1.0 uses XPath 1.0, while XSLT 2.0 uses XPath 2.0. XSLT 3.0 will work with either XPath 3.0 or 3.1. In the case of 1.0 and 2.0, the XSLT and XPath specifications were published on the same date. With 3.0, however, they were no longer synchronized; XPath 3.0 became a Recommendation in April 2014, followed by XPath 3.1 in February 2017; XSLT 3.0 followed in June 2017. XSLT functionalities overlap with those ofXQuery, which was initially conceived as a query language for large collections of XML documents. The XSLT 2.0 and XQuery 1.0 standards were developed by separate working groups withinW3C, working together to ensure a common approach where appropriate. They share the same data model, type system, and function library, and both includeXPath2.0 as a sublanguage. The two languages, however, are rooted in different traditions and serve the needs of different communities. XSLT was primarily conceived as a stylesheet language whose primary goal was to render XML for the human reader on screen, onthe web(as aweb template language), or on paper. XQuery was primarily conceived as adatabase query languagein the tradition ofSQL. Because the two languages originate in different communities, XSLT is stronger in its handling of narrative documents with more flexible structure, while XQuery is stronger in its data handling, for example when performing relational joins.[20] The<output>element can optionally take the attributemedia-type, which allows one to set themedia type(or MIME type) for the resulting output, for example:<xsl:output output="xml" media-type="application/xml"/>. The XSLT 1.0 recommendation recommends the more general attribute typestext/xmlandapplication/xmlsince for a long time there was no registered media type for XSLT. During this timetext/xslbecame the de facto standard. In XSLT 1.0 it was not specified how themedia-typevalues should be used. With the release of the XSLT 2.0, the W3C recommended in 2007 the registration of the MIME media typeapplication/xslt+xml[21]and it was later registered with theInternet Assigned Numbers Authority.[22] Pre-1.0 working drafts of XSLT usedtext/xslin their embedding examples, and this type was implemented and continued to be promoted by Microsoft in Internet Explorer[23]and MSXML circa 2012. It is also widely recognized in thexml-stylesheetprocessing instruction by other browsers. In practice, therefore, users wanting to control transformation in the browser using this processing instruction were obliged to use this unregistered media type.[24] These examples use the following incoming XML document: This XSLT stylesheet provides templates to transform the XML document: Its evaluation results in a new XML document, having another structure: Processing the following example XSLT file with the XML input file shown above results in the followingXHTML(whitespacehas been adjusted here for clarity): This XHTML generates the output below when rendered in a web browser. In order for a web browser to be able to apply an XSL transformation to an XML document on display, an XML stylesheet processing instruction can be inserted into XML. So, for example, if the stylesheet in Example 2 above were available as "example2.xsl", the following instruction could be added to the original incoming XML:[25] In this example,text/xslis technically incorrect according to the W3C specifications[25](which say the type should beapplication/xslt+xml), but it is the only media type that is widely supported across browsers as of 2009, and the situation is unchanged in 2021. Most early XSLT processors were interpreters. More recently, code generation is increasingly common, using portable intermediate languages (such asJava bytecodeor .NETCommon Intermediate Language) as the target. However, even the interpretive products generally offer separate analysis and execution phases, allowing an optimized expression tree to be created in memory and reused to perform multiple transformations. This gives substantial performance benefits in online publishing applications, where the same transformation is applied many times per second to different source documents.[42]This separation is reflected in the design of XSLT processing APIs (such asJAXP). Early XSLT processors had very few optimizations. Stylesheet documents were read intoDocument Object Modelsand the processor would act on them directly.XPathengines were also not optimized. Increasingly, however, XSLT processors use optimization techniques found in functional programming languages and database query languages, such as static rewriting of an expression tree (e.g., to move calculations out of loops), and lazy pipelined evaluation to reduce thememory footprintof intermediate results (and allow "early exit" when the processor can evaluate an expression such asfollowing-sibling::*[1]without a complete evaluation of all subexpressions). Many processors also use tree representations that are significantly more efficient (in both space and time)[43]than general-purpose DOM implementations. In June 2014, Debbie Lockett andMichael Kayintroduced an open-source benchmarking framework for XSLT processors called XT-Speedo.[44]	https://en.wikipedia.org/wiki/XSLT
Acascading failureis a failure in asystemofinterconnectedparts in which the failure of one or few parts leads to the failure of other parts, growing progressively as a result ofpositive feedback. This can occur when a single part fails, increasing the probability that other portions of the system fail.[1][2][3]Such a failure may happen in many types of systems, including power transmission, computer networking, finance, transportation systems, organisms, the human body, and ecosystems. Cascading failures may occur when one part of the system fails. When this happens, other parts must then compensate for the failed component. This in turn overloads these nodes, causing them to fail as well, prompting additional nodes to fail one after another. Cascading failure is common inpower gridswhen one of the elements fails (completely or partially) and shifts its load to nearby elements in the system. Those nearby elements are then pushed beyond their capacity so they become overloaded and shift their load onto other elements. Cascading failure is a common effect seen inhigh voltagesystems, where asingle point of failure(SPF) on a fully loaded or slightly overloaded system results in a sudden spike across all nodes of the system. This surge current can induce the already overloaded nodes into failure, setting off more overloads and thereby taking down the entire system in a very short time. This failure process cascades through the elements of the system like a ripple on a pond and continues until substantially all of the elements in the system are compromised and/or the system becomes functionally disconnected from the source of its load. For example, under certain conditions a large power grid can collapse after the failure of a single transformer. Monitoring the operation of a system, inreal-time, and judicious disconnection of parts can help stop a cascade. Another common technique is to calculate a safety margin for the system by computer simulation of possible failures, to establish safe operating levels below which none of the calculated scenarios is predicted to cause cascading failure, and to identify the parts of the network which are most likely to cause cascading failures.[4] One of the primary problems with preventing electrical grid failures is that the speed of the control signal is no faster than the speed of the propagating power overload, i.e. since both the control signal and the electrical power are moving at the same speed, it is not possible to isolate the outage by sending a warning ahead to isolate the element. Cascading failure caused the followingpower outages: Cascading failures can also occur incomputer networks(such as theInternet) in whichnetwork trafficis severely impaired or halted to or between larger sections of the network, caused by failing or disconnected hardware or software. In this context, the cascading failure is known by the termcascade failure. A cascade failure can affect large groups of people and systems. The cause of a cascade failure is usually the overloading of a single, crucialrouteror node, which causes the node to go down, even briefly. It can also be caused by taking a node down for maintenance or upgrades. In either case, traffic isroutedto or through another (alternative) path. This alternative path, as a result, becomes overloaded, causing it to go down, and so on. It will also affect systems which depend on the node for regular operation. The symptoms of a cascade failure include:packet lossand highnetwork latency, not just to single systems, but to whole sections of a network or the internet. The high latency and packet loss is caused by the nodes that fail to operate due tocongestion collapse, which causes them to still be present in the network but without much or any useful communication going through them. As a result, routes can still be considered valid, without them actually providing communication. If enough routes go down because of a cascade failure, a complete section of the network or internet can become unreachable. Although undesired, this can help speed up the recovery from this failure as connections will time out, and other nodes will give up trying to establish connections to the section(s) that have become cut off, decreasing load on the involved nodes. A common occurrence during a cascade failure is a walking failure, where sections go down, causing the next section to fail, after which the first section comes back up. This ripple can make several passes through the same sections or connecting nodes before stability is restored. Cascade failures are a relatively recent development, with the massive increase in traffic and the high interconnectivity between systems and networks. The term was first applied in this context in the late 1990s by a Dutch IT professional and has slowly become a relatively common term for this kind of large-scale failure.[citation needed] Network failures typically start when a single network node fails. Initially, the traffic that would normally go through the node is stopped. Systems and users get errors about not being able to reach hosts. Usually, the redundant systems of an ISP respond very quickly, choosing another path through a different backbone. The routing path through this alternative route is longer, with morehopsand subsequently going through more systems that normally do not process the amount of traffic suddenly offered. This can cause one or more systems along the alternative route to go down, creating similar problems of their own. Related systems are also affected in this case. As an example,DNSresolution might fail and what would normally cause systems to be interconnected, might break connections that are not even directly involved in the actual systems that went down. This, in turn, may cause seemingly unrelated nodes to develop problems, that can cause another cascade failure all on its own. In December 2012, a partial loss (40%) ofGmailservice occurred globally, for 18 minutes. This loss of service was caused by a routine update of load balancing software which contained faulty logic—in this case, the error was caused by logic using an inappropriate 'all' instead of the more appropriate 'some'.[5]The cascading error was fixed by fully updating a single node in the network instead of partially updating all nodes at one time. Certain load-bearing structures with discrete structural components can be subject to the "zipper effect", where the failure of a single structural member increases the load on adjacent members. In the case of theHyatt Regency walkway collapse, a suspended walkway (which was already overstressed due to an error in construction) failed when a single vertical suspension rod failed, overloading the neighboring rods which failed sequentially (i.e. like azipper). A bridge that can have such a failure is called fracture critical, and numerous bridge collapses have been caused by the failure of a single part. Properly designed structures use an adequatefactor of safetyand/or alternate load paths to prevent this type of mechanical cascade failure.[6] Fracture cascade is a phenomenon in the context of geology and describes triggering a chain reaction of subsequent fractures by a single fracture.[7]The initial fracture leads to the propagation of additional fractures, causing a cascading effect throughout the material. Fracture cascades can occur in various materials, including rocks, ice, metals, and ceramics.[8]A common example is the bending of dryspaghetti, which in most cases breaks into more than 2 pieces, as first observed byRichard Feynman.[8] In the context ofosteoporosis, a fracture cascade is the increased risk of subsequent bone fractures after an initial one.[9] Biochemical cascadesexist in biology, where a small reaction can have system-wide implications. One negative example isischemic cascade, in which a smallischemicattack releasestoxinswhich kill off far more cells than the initial damage, resulting in more toxins being released. Current research is to find a way to block this cascade instrokepatients to minimize the damage. In the study of extinction, sometimes the extinction of one species will cause many other extinctions to happen. Such a species is known as akeystone species. Another example is theCockcroft–Walton generator, which can also experience cascade failures wherein one faileddiodecan result in all the diodes failing in a fraction of a second. Yet another example of this effect in a scientific experiment was theimplosionin 2001 of several thousand fragile glass photomultiplier tubes used in theSuper-Kamiokandeexperiment, where the shock wave caused by the failure of a single detector appears to have triggered the implosion of the other detectors in a chain reaction. Infinance, the risk of cascading failures of financial institutions is referred to assystemic risk:the failure of one financial institution may cause other financial institutions (itscounterparties) to fail, cascading throughout the system. Institutions that are believed to pose systemic risk are deemed either "too big to fail" (TBTF) or "too interconnected to fail" (TICTF), depending on why they appear to pose a threat. Note however that systemic risk is not due to individual institutions per se, but due to the interconnections. Frameworks to study and predict the effects of cascading failures have been developed in the research literature.[10][11][12] A related (though distinct) type of cascading failure in finance occurs in the stock market, exemplified by the2010 Flash Crash.[12] Diverseinfrastructuressuch aswater supply,transportation, fuel andpower stationsare coupled together and depend on each other for functioning, see Fig. 1. Owing to this coupling, interdependent networks are extremely sensitive to random failures, and in particular totargeted attacks, such that a failure of a small fraction of nodes in one network can trigger an iterative cascade of failures in several interdependent networks.[13][14]Electrical blackoutsfrequently result from a cascade of failures between interdependent networks, and the problem has been dramatically exemplified by the several large-scale blackouts that have occurred in recent years. Blackouts are a fascinating demonstration of the important role played by the dependencies between networks. For example, the2003 Italy blackoutresulted in a widespread failure of therailway network,health care systems, andfinancial servicesand, in addition, severely influenced thetelecommunication networks. The partial failure of the communication system in turn further impaired theelectrical gridmanagement system, thus producing a positive feedback on the power grid.[15]This example emphasizes how inter-dependence can significantly magnify the damage in an interacting network system. A model for cascading failures due to overload propagation is the Motter–Lai model.[16]	https://en.wikipedia.org/wiki/Cascading_failure
Incomputer programming,cohesionrefers to thedegree to which the elements inside amodulebelong together.[1]In one sense, it is a measure of the strength of relationship between themethodsand data of aclassand some unifying purpose or concept served by that class. In another sense, it is a measure of the strength of relationship between the class's methods and data. Cohesion is anordinaltype of measurement and is usually described as “high cohesion” or “low cohesion”. Modules with high cohesion tend to be preferable, because high cohesion is associated with several desirable software traits includingrobustness, reliability,reusability, and understandability. In contrast, low cohesion is associated with undesirable traits such as being difficult to maintain, test, reuse, or understand. Cohesion is often contrasted withcoupling. High cohesion often correlates withloose coupling, and vice versa.[2]Thesoftware metricsof coupling and cohesion were invented byLarry Constantinein the late 1960s as part ofStructured Design, based on characteristics of “good” programming practices that reduced maintenance and modification costs. Structured Design, cohesion and coupling were published in the articleStevens, Myers & Constantine(1974)[3]and the bookYourdon & Constantine(1979).[1]The latter two subsequently became standard terms insoftware engineering. Inobject-oriented programming, a class is said to have high cohesion if the methods that serve the class are similar in many aspects.[4]In a highly cohesive system, code readability andreusabilityis increased, while complexity is kept manageable. Cohesion is increased if: Advantages of high cohesion (or "strong cohesion") are: While in principle a module can have perfect cohesion by only consisting of a single, atomic element – having a single function, for example – in practice complex tasks are not expressible by a single, simple element. Thus a single-element module has an element that is either too complicated to accomplish a task, or too narrow and thus tightlycoupledto other modules. Thus cohesion is balanced with both unit complexity and coupling. Cohesion is a qualitative measure, meaning that the source code is examined using arubricto determine a classification. Cohesion types, from the worst to the best, are as follows: Although cohesion is a ranking type of scale, the ranks do not indicate a steady progression of improved cohesion. Studies byLarry Constantine,Edward Yourdon, andSteve McConnell[5]indicate that the first two types of cohesion are inferior, communicational and sequential cohesion are very good, and functional cohesion is superior.	https://en.wikipedia.org/wiki/Cohesion_(computer_science)
Connascenceis a software design metric introduced by Meilir Page-Jones that quantifies the degree and type of dependency between software components, evaluating their strength (difficulty of change) and locality (proximity in the codebase). It can be categorized as static (analyzable at compile-time) or dynamic (detectable at runtime) and includes forms such as Connascence of Name, Type, and Position, each representing different dependency characteristics and levels of fragility.[1][2] Coupling describes the degree and nature of dependency between software components, focusing on what they share (e.g., data, control flow, technology) and how tightly they are bound. It evaluates two key dimensions: strength, which measures how difficult it is to change the dependency, and scope (or visibility), which indicates how widely the dependency is exposed across modules or boundaries. Traditional coupling types typically include content coupling, common coupling, control coupling, stamp coupling, external coupling, and data coupling.[1][3][2] Connascence, introduced by Meilir Page-Jones, provides a systematic framework for analyzing and measuring coupling dependencies. It evaluates dependencies based on three dimensions: strength, which measures the effort required to refactor or modify the dependency; locality, which considers how physically or logically close dependent components are in the codebase; and degree, which measures how many components are affected by the dependency. Connascence can be categorized into static (detectable at compile-time) and dynamic (detectable at runtime) forms. Static connascence refers to compile-time dependencies, such as method signatures, while dynamic connascence refers to runtime dependencies, which can manifest in forms like connascence of timing, values, or algorithm.[1][3][2] Each coupling flavor can exhibit multiple types of connascence, a specific type, or, in rare cases, none at all, depending on how the dependency is implemented. Common types of connascence include connascence of name, type, position, and meaning. Certain coupling types naturally align with specific connascence types; for example, data coupling often involves connascence of name or type. However, not every combination of coupling and connascence is practically meaningful. Dependencies relying on parameter order in a method signature demonstrate connascence of position, which is fragile and difficult to refactor because reordering parameters breaks the interface. In contrast, connascence of name, which relies on field or parameter names, is generally more resilient to change. Connascence types themselves exhibit a natural hierarchy of strength, with connascence of name typically considered weaker than connascence of meaning.[1][3][2] Dependencies spanning module boundaries or distributed systems typically have higher coordination costs, increasing the difficulty of refactoring and propagating changes across distant boundaries. Modern practices, such as dependency injection and interface-based programming, are often employed to reduce coupling strength and improve the maintainability of dependencies.[1][3][2] While coupling identifies what is shared between components, connascence evaluates how those dependencies behave, how changes propagate, and how difficult they are to refactor. Strength, locality, and degree are interrelated; dependencies with high strength, wide scope, and spanning distant boundaries are significantly harder to refactor and maintain. Together, coupling provides a high-level overview of dependency relationships, while connascence offers a granular framework for analyzing dependency strength, locality, degree, and resilience to change, supporting the design of maintainable and robust systems.[1][3][2]	https://en.wikipedia.org/wiki/Connascence_(computer_programming)
Insoftware engineering,couplingis the degree of interdependence between softwaremodules, a measure of how closely connected two routines or modules are,[1]and the strength of the relationships between modules.[2]Coupling is not binary but multi-dimensional.[3] Coupling is usually contrasted withcohesion.Low couplingoften correlates with high cohesion, and vice versa. Low coupling is often thought to be a sign of a well-structuredcomputer systemand a good design, and when combined with high cohesion, supports the general goals of highreadabilityandmaintainability.[citation needed] Thesoftware quality metricsof coupling and cohesion were invented byLarry Constantinein the late 1960s as part of astructured design, based on characteristics of “good” programming practices that reduced maintenance and modification costs. Structured design, including cohesion and coupling, were published in the articleStevens, Myers & Constantine(1974)[4]and the bookYourdon & Constantine(1979),[5]and the latter subsequently became standard terms. Coupling can be "low" (also "loose" and "weak") or "high" (also "tight" and "strong"). Some types of coupling, in order of highest to lowest coupling, are as follows: A module here refers to a subroutine of any kind, i.e. a set of one or more statements having a name and preferably its own set of variable names. In recent work various other coupling concepts have been investigated and used as indicators for different modularization principles used in practice.[7] The goal of defining and measuring this type of coupling is to provide a run-time evaluation of a software system. It has been argued that static coupling metrics lose precision when dealing with an intensive use of dynamic binding or inheritance.[8]In the attempt to solve this issue, dynamic coupling measures have been taken into account. This kind of a coupling metric considers the conceptual similarities between software entities using, for example, comments and identifiers and relying on techniques such aslatent semantic indexing(LSI). Logical coupling (or evolutionary coupling or change coupling) analysis exploits the release history of a software system to find change patterns among modules or classes: e.g., entities that are likely to be changed together or sequences of changes (a change in a class A is always followed by a change in a class B). According to Gregor Hohpe, coupling is multi-dimensional:[3] Tightly coupled systems tend to exhibit the following developmental characteristics, which are often seen as disadvantages: Whether loosely or tightly coupled, a system's performance is often reduced by message and parameter creation, transmission, translation (e.g. marshaling) and message interpretation (which might be a reference to a string, array or data structure), which require less overhead than creating a complicated message such as aSOAPmessage. Longer messages require more CPU and memory to produce. To optimize runtime performance, message length must be minimized and message meaning must be maximized. One approach to decreasing coupling isfunctional design, which seeks to limit the responsibilities of modules along functionality. Coupling increases between two classesAandBif: Low coupling refers to a relationship in which one module interacts with another module through a simple and stable interface and does not need to be concerned with the other module's internal implementation (seeInformation Hiding). Systems such asCORBAorCOMallow objects to communicate with each other without having to know anything about the other object's implementation. Both of these systems even allow for objects to communicate with objects written in other languages. Coupling describes the degree and nature of dependency between software components, focusing on what they share (e.g., data, control flow, technology) and how tightly they are bound. It evaluates two key dimensions: strength, which measures how difficult it is to change the dependency, and scope (or visibility), which indicates how widely the dependency is exposed across modules or boundaries. Traditional coupling types typically include content coupling, common coupling, control coupling, stamp coupling, external coupling, and data coupling.[9][10][11] Connascence, introduced by Meilir Page-Jones, provides a systematic framework for analyzing and measuring coupling dependencies. It evaluates dependencies based on three dimensions: strength, which measures the effort required to refactor or modify the dependency; locality, which considers how physically or logically close dependent components are in the codebase; and degree, which measures how many components are affected by the dependency. Connascence can be categorized into static (detectable at compile-time) and dynamic (detectable at runtime) forms. Static connascence refers to compile-time dependencies, such as method signatures, while dynamic connascence refers to runtime dependencies, which can manifest in forms like connascence of timing, values, or algorithm.[9][10][11] Each coupling flavor can exhibit multiple types of connascence, a specific type, or, in rare cases, none at all, depending on how the dependency is implemented. Common types of connascence include connascence of name, type, position, and meaning. Certain coupling types naturally align with specific connascence types; for example, data coupling often involves connascence of name or type. However, not every combination of coupling and connascence is practically meaningful. Dependencies relying on parameter order in a method signature demonstrate connascence of position, which is fragile and difficult to refactor because reordering parameters breaks the interface. In contrast, connascence of name, which relies on field or parameter names, is generally more resilient to change. Connascence types themselves exhibit a natural hierarchy of strength, with connascence of name typically considered weaker than connascence of meaning.[9][10][11] Dependencies spanning module boundaries or distributed systems typically have higher coordination costs, increasing the difficulty of refactoring and propagating changes across distant boundaries. Modern practices, such as dependency injection and interface-based programming, are often employed to reduce coupling strength and improve the maintainability of dependencies.[9][10][11] While coupling identifies what is shared between components, connascence evaluates how those dependencies behave, how changes propagate, and how difficult they are to refactor. Strength, locality, and degree are interrelated; dependencies with high strength, wide scope, and spanning distant boundaries are significantly harder to refactor and maintain. Together, coupling provides a high-level overview of dependency relationships, while connascence offers a granular framework for analyzing dependency strength, locality, degree, and resilience to change, supporting the design of maintainable and robust systems.[9][10][11] Coupling andcohesionare terms which occur together very frequently. Coupling refers to the interdependencies between modules, while cohesion describes how related the functions within a single module are. Low cohesion implies that a given module performs tasks which are not very related to each other and hence can create problems as the module becomes large. Coupling in Software Engineering[12]describes a version of metrics associated with this concept. For data and control flow coupling: For global coupling: For environmental coupling: Coupling(C)=1−1di+2×ci+do+2×co+gd+2×gc+w+r{\displaystyle \mathrm {Coupling} (C)=1-{\frac {1}{d_{i}+2\times c_{i}+d_{o}+2\times c_{o}+g_{d}+2\times g_{c}+w+r}}} Coupling(C)makes the value larger the more coupled the module is. This number ranges from approximately 0.67 (low coupling) to 1.0 (highly coupled) For example, if a module has only a single input and output data parameter C=1−11+0+1+0+0+0+1+0=1−13=0.67{\displaystyle C=1-{\frac {1}{1+0+1+0+0+0+1+0}}=1-{\frac {1}{3}}=0.67} If a module has 5 input and output data parameters, an equal number of control parameters, and accesses 10 items of global data, with a fan-in of 3 and a fan-out of 4, C=1−15+2×5+5+2×5+10+0+3+4=0.98{\displaystyle C=1-{\frac {1}{5+2\times 5+5+2\times 5+10+0+3+4}}=0.98}	https://en.wikipedia.org/wiki/Coupling_(computer_science)
TheLaw of Demeter(LoD) orprinciple of least knowledgeis a design guideline for developingsoftware, particularlyobject-oriented programs. In its general form, the LoD is a specific case ofloose coupling. The guideline was proposed by Ian Holland atNortheastern Universitytowards the end of 1987,[1]and the following three recommendations serve as a succinct summary:[2] The fundamental notion is that a given object should assume as little as possible about the structure or properties of anything else (including its subcomponents), in accordance with the principle of "information hiding". It may be viewed as a corollary to theprinciple of least privilege, which dictates that a module possess only the information and resources necessary for its legitimate purpose. It is so named for its origin in theDemeter Project, anadaptive programmingandaspect-oriented programmingeffort. The project was named in honor ofDemeter, "distribution-mother" and the Greekgoddessofagriculture, to signify abottom-upphilosophy of programming which is also embodied in the law itself.[3][non-primary source needed] The law of Demeter dates back to 1987 when it was first proposed by Ian Holland, who was working on the Demeter Project. This project was the birthplace of a lot ofaspect-oriented programming(AOP) principles. A quote in one of the remainders of the project seems to clarify the origins of the name:[4] Demeter The Greek goddess of Agriculture. The Demeter project was named after Demeter because we were working on a hardware description language Zeus and we were looking for a tool to simplify the implementation of Zeus. We were looking for a tool name related to Zeus and we chose a sister of Zeus: Demeter. We later promoted the idea that Demeter-style software development is about growing software as opposed to building software. We introduced the concept of a growth plan which is basically a sequence of more and more complex UML class diagrams. Growth plans are useful for building systems incrementally. An objectacan request a service (call a method) of an object instanceb, but objectashould not "reach through" objectbto access yet another object,c, to request its services. Doing so would mean that objectaimplicitly requires greater knowledge of objectb's internal structure. Instead,b's interface should be modified if necessary so it can directly serve objecta's request, propagating it to any relevant subcomponents. Alternatively,amight have a direct reference to objectcand make the request directly to that. If the law is followed, only objectbknows its own internal structure. More formally, the Law of Demeter for functions requires that a methodmof an objectamay only invoke the methods of the following kinds of objects:[5] In particular, an object should avoid invoking methods of an object returned by another method. For many modern object-oriented languages that use a dot as field identifier, the law can be stated simply as "use only one dot".[6]That is, the codea.m().n()breaks the law wherea.m()does not. As ananalogy, when one wants a dog to walk, one does not command the dog's legs to walk directly; instead, one commands the dog which then commands its own legs. The advantage of following the Law of Demeter is that the resulting software tends to be moremaintainableandadaptable. Since objects areless dependenton the internal structure of other objects, object implementation can bechangedwithout reworking their callers. Basili et al.[7]published experimental results in 1996 suggesting that a lowerResponse For a Class(RFC, the number of methods potentially invoked in response to calling a method of that class) can reduce the probability ofsoftware bugs. Following the Law of Demeter can result in a lower RFC. However, the results also suggest that an increase inWeighted Methods per Class[8](WMC, the number of methods defined in each class) can increase the probability of software bugs. Following the Law of Demeter can also result in a higher WMC. Amultilayered architecturecan be considered to be a systematic mechanism for implementing the Law of Demeter in a software system. In a layered architecture, code within eachlayercan only make calls to code within the layer and code within the next layer down. "Layer skipping" would violate the layered architecture. Although the LoD increases the adaptiveness of a software system, it may result in having to write manywrapper methodsto propagate calls to components; in some cases, this can add noticeable time and space overhead.[7][9][10] At the method level, the LoD leads to narrow interfaces, giving access to only as much information as it needs to do its job, as each method needs to know about a small set of methods of closely related objects.[11]On the other hand, at the class level, if the LoD is not used correctly, wide (i.e., enlarged) interfaces may be developed that require introducing many auxiliary methods.[9][10]This is due to poor design rather than a consequence of the LoD per se. If a wrapper method is being used, it means that the object being called through the wrapper should have been a dependency in the calling class. One proposed solution to the problem of enlarged class interfaces is theaspect-orientedapproach,[12]where the behavior of the method is specified as an aspect at a high level of abstraction. The wide interfaces are managed through a language that specifies implementations. Both the traversal strategy and the adaptive visitor use only a minimal set of classes that participate in the operation, and the information about the connections between these classes is abstracted out.	https://en.wikipedia.org/wiki/Law_of_Demeter
Incomputer science,separation of concerns(sometimes abbreviated asSoC) is a design principle for separating acomputer programinto distinct sections. Each section addresses a separateconcern, a set of information that affects the code of a computer program. A concern can be as general as "the details of the hardware for an application", or as specific as "the name of which class toinstantiate". A program that embodies SoC well is called amodular[1]program. Modularity, and hence separation of concerns, is achieved byencapsulatinginformation inside a section of code that has a well-defined interface. Encapsulation is a means ofinformation hiding.[2]Layered designs or packaging by feature in information systems are another embodiment of separation of concerns (e.g., presentation layer, business logic layer, data access layer, persistence layer).[3] Separation of concerns results in more degrees of freedom for some aspect of the program's design, deployment, or usage. Common among these is increased freedom for simplification and maintenance of code. When concerns are well-separated, there are more opportunities for module upgrade, reuse, and independent development. Hiding the implementation details of modules behind an interface enables improving or modifying a single concern's section of code without having to know the details of other sections and without having to make corresponding changes to those other sections. Modules can also expose different versions of an interface, which increases the freedom to upgrade a complex system in piecemeal fashion without interim loss of functionality.[4] Separation of concerns is a form ofabstraction. As with most abstractions, separating concerns means adding additional code interfaces, generally creating more code to be executed. The extra code can result in higher computation costs in some cases, but in other cases also can lead to reuse of more optimized code. So despite the many benefits of well-separated concerns, there may be an associated execution penalty.[4] The mechanisms for modular or object-oriented programming that are provided by aprogramming languageare mechanisms that allow developers to provide SoC.[5]For example,object-oriented programminglanguages such asC#,C++,Delphi, andJavacan separate concerns intoobjects, and architecturaldesign patternslikeMVCorMVPcanseparate presentation and the data-processing (model) from content.Service-oriented designcan separate concerns intoservices.Procedural programminglanguages such asCandPascalcan separate concerns intoproceduresorfunctions.Aspect-oriented programminglanguages can separate concerns intoaspectsandobjects. Separation of concerns is an important design principle in many other areas as well, such asurban planning,architectureandinformation design.[6]The goal is to more effectively understand, design, and manage complex interdependent systems, so that functions can be reused, optimized independently of other functions, and insulated from the potential failure of other functions. Common examples include separating a space into rooms, so that activity in one room does not affect people in other rooms, and keeping the stove on one circuit and the lights on another, so that overload by the stove does not turn the lights off. The example with rooms shows encapsulation, where information inside one room, such as how messy it is, is not available to the other rooms, except through the interface, which is the door. The example with circuits demonstrates that activity inside one module, which is a circuit with consumers of electricity attached, does not affect activity in a different module, so each module is not concerned with what happens in the other. The termseparation of concernswas probably coined byEdsger W. Dijkstrain his 1974 paper "On the role of scientific thought".[7] Let me try to explain to you, what to my taste is characteristic for all intelligent thinking. It is, that one is willing to study in depth an aspect of one's subject matter in isolation for the sake of its own consistency, all the time knowing that one is occupying oneself only with one of the aspects. We know that a program must be correct and we can study it from that viewpoint only; we also know that it should be efficient and we can study its efficiency on another day, so to speak. In another mood we may ask ourselves whether, and if so: why, the program is desirable. But nothing is gained—on the contrary!—by tackling these various aspects simultaneously. It is what I sometimes have called "the separation of concerns", which, even if not perfectly possible, is yet the only available technique for effective ordering of one's thoughts, that I know of. This is what I mean by "focusing one's attention upon some aspect": it does not mean ignoring the other aspects, it is just doing justice to the fact that from this aspect's point of view, the other is irrelevant. It is being one- and multiple-track minded simultaneously. Fifteen years later, it was evident the termseparation of concernswas becoming an accepted idea. In 1989, Chris Reade wrote a book titledElements of Functional Programming[8]that describes separation of concerns: The programmer is having to do several things at the same time, namely, Reade continues to say, Ideally, the programmer should be able to concentrate on the first of the three tasks (describing what is to be computed) without being distracted by the other two, more administrative, tasks. Clearly, administration is important, but by separating it from the main task we are likely to get more reliable results and we can ease the programming problem by automating much of the administration. The separation of concerns has other advantages as well. For example, program proving becomes much more feasible when details of sequencing and memory management are absent from the program. Furthermore, descriptions of what is to be computed should be free of such detailed step-by-step descriptions of how to do it, if they are to be evaluated with different machine architectures. Sequences of small changes to a data object held in a store may be an inappropriate description of how to compute something when a highly parallel machine is being used with thousands of processors distributed throughout the machine and local rather than global storage facilities. Automating the administrative aspects means that the language implementor has to deal with them, but he/she has far more opportunity to make use of very different computation mechanisms with different machine architectures. Separation of concerns is crucial to the design of the Internet. In theInternet protocol suite, great efforts have been made to separate concerns into well-definedlayers. This allows protocol designers to focus on the concerns in one layer, and ignore the other layers. The Application Layer protocolSMTP, for example, is concerned about all the details of conducting an email session over a reliable transport service (usuallyTCP), but not in the least concerned about how the transport service makes that service reliable. Similarly, TCP is not concerned about the routing of data packets, which is handled at theInternet layer. HTML,CSS, andJavaScriptare complementary languages used in the development of web pages and websites. HTML is mainly used for organization of webpage content, CSS is used for definition of content presentation style, and JavaScript defines how the content interacts and behaves with the user. Historically, this was not the case: prior to the introduction of CSS, HTML performed both duties of defining semantics and style. Subject-oriented programmingallows separate concerns to be addressed as separate software constructs, each on an equal footing with the others. Each concern provides its own class-structure into which the objects in common are organized, and contributes state and methods to the composite result where they cut across one another. Correspondence rules describe how the classes and methods in the various concerns are related to each other at points where they interact, allowing composite behavior for a method to be derived from several concerns.Multi-dimensional separation of concernsallows the analysis and composition of concerns to be manipulated as a multi-dimensional "matrix" in which each concern provides a dimension in which different points of choice are enumerated, with the cells of the matrix occupied by the appropriate software artifacts. Aspect-oriented programmingallowscross-cutting concernsto be addressed as primary concerns. For example, most programs require some form ofsecurityandlogging. Security and logging are often secondary concerns, whereas the primary concern is often on accomplishing business goals. However, when designing a program, its security must be built into the design from the beginning instead of being treated as a secondary concern. Applying security afterwards often results in an insufficient security model that leaves too many gaps for future attacks. This may be solved with aspect-oriented programming. For example, an aspect may be written to enforce that calls to a certain API are always logged, or that errors are always logged when an exception is thrown, regardless of whether the program's procedural code handles the exception or propagates it.[9] Incognitive scienceandartificial intelligence, it is common to refer to David Marr'slevels of analysis. At any given time, a researcher may be focusing on (1) what some aspect of intelligence needs to compute, (2) what algorithm it employs, or (3) how that algorithm is implemented in hardware. This separation of concerns is similar to theinterface/implementation distinction in software and hardware engineering. Innormalized systemsseparation of concerns is one of the four guiding principles. Adhering to this principle is one of the tools that helps reduce the combinatorial effects that, over time, get introduced in software that is being maintained. In normalized systems separation of concerns is actively supported by the tools. Separation of concerns can be implemented and enforced viapartial classes.[10]	https://en.wikipedia.org/wiki/Separation_of_concerns
Insoftware engineering,service-oriented architecture(SOA) is an architectural style that focuses on discrete services instead of amonolithic design.[1]SOA is a good choice forsystem integration.[2]By consequence, it is also applied in the field ofsoftware designwhere services are provided to the other components byapplication components, through acommunication protocolover a network. A service is a discrete unit of functionality that can be accessed remotely and acted upon and updated independently, such as retrieving a credit card statement online. SOA is also intended to be independent of vendors, products and technologies.[3] Service orientation is a way of thinking in terms of services and service-based development and the outcomes of services.[1] A service has four properties according to one of many definitions of SOA:[4] Different services can be used in conjunction as aservice meshto provide the functionality of a largesoftware application,[6]a principle SOA shares withmodular programming. Service-oriented architecture integrates distributed, separately maintained and deployed software components. It is enabled by technologies and standards that facilitate components' communication and cooperation over a network, especially over an IP network. SOA is related to the idea of an API (application programming interface), an interface or communication protocol between different parts of a computer program intended to simplify the implementation and maintenance of software. An API can be thought of as the service, and the SOA the architecture that allows the service to operate. Note that Service-Oriented Architecture must not be confused with Service Based Architecture as those are two different architectural styles.[7] In SOA, services use protocols that describe how theypassand parse messages using descriptionmetadata. This metadata describes both the functional characteristics of the service and quality-of-service characteristics. Service-oriented architecture aims to allow users to combine large chunks of functionality to form applications which are built purely from existing services and combining them in an ad hoc manner. A service presents a simple interface to the requester that abstracts away the underlying complexity acting as a black box. Further users can also access these independent services without any knowledge of their internal implementation.[8] The related buzzwordservice-orientationpromotesloose couplingbetween services. SOA separates functions into distinct units, or services,[9]which developers make accessible over a network in order to allow users to combine and reuse them in the production of applications. These services and their corresponding consumers communicate with each other by passing data in a well-defined, shared format, or by coordinating an activity between two or more services.[10] SOA can be seen as part of the continuum which ranges from the older concept ofdistributed computing[9][11]andmodular programming, through SOA, and on to practices ofmashups,SaaS, andcloud computing(which some see as the offspring of SOA).[12] There are no industry standards relating to the exact composition of a service-oriented architecture, although many industry sources have published their own principles. Some of these[13][14][15]include the following: Each SOA building block can play any of the three roles: The service consumer–provider relationship is governed by astandardized service contract,[19]which has a business part, a functional part and a technical part. Service composition patternshave two broad, high-level architectural styles:choreography and orchestration. Lower level enterprise integration patterns that are not bound to a particular architectural style continue to be relevant and eligible in SOA design.[20][21][22] Service-oriented architecture can be implemented withweb servicesorMicroservices.[23]This is done to make the functional building-blocks accessible over standard Internet protocols that are independent of platforms and programming languages. These services can represent either new applications or just wrappers around existing legacy systems to make them network-enabled.[24] Implementers commonly build SOAs using web services standards. One example isSOAP, which has gained broad industry acceptance after the recommendation of Version 1.2 from the W3C[25](World Wide Web Consortium) in 2003. These standards (also referred to asweb service specifications) also provide greater interoperability and some protection from lock-in to proprietary vendor software. One can, however, also implement SOA using any other service-based technology, such asJini,CORBA,Internet Communications Engine,REST, orgRPC. Architectures can operate independently of specific technologies and can therefore be implemented using a wide range of technologies, including: Implementations can use one or more of these protocols and, for example, might use a file-system mechanism to communicate data following a defined interface specification between processes conforming to the SOA concept. The key is independent services with defined interfaces that can be called to perform their tasks in a standard way, without a service having foreknowledge of the calling application, and without the application having or needing knowledge of how the service actually performs its tasks. SOA enables the development of applications that are built by combining loosely coupled andinteroperableservices. These services inter-operate based on a formal definition (or contract, e.g., WSDL) that is independent of the underlying platform and programming language. The interface definitionhides the implementationof the language-specific service. SOA-based systems can therefore function independently of development technologies and platforms (such as Java, .NET, etc.). Services written in C# running on .NET platforms and services written in Java running onJava EEplatforms, for example, can both be consumed by a common composite application (or client). Applications running on either platform can also consume services running on the other as web services that facilitate reuse. Managed environments can also wrap COBOL legacy systems and present them as software services..[26] High-level programming languagessuch asBPELand specifications such asWS-CDLandWS-Coordinationextend the service concept by providing a method of defining and supporting orchestration of fine-grained services into more coarse-grained business services, which architects can in turn incorporate into workflows and business processes implemented incomposite applicationsorportals. Service-oriented modelingis an SOA framework that identifies the various disciplines that guide SOA practitioners to conceptualize, analyze, design, and architect their service-oriented assets. TheService-oriented modeling framework (SOMF)offers a modeling language and a work structure or "map" depicting the various components that contribute to a successful service-oriented modeling approach. It illustrates the major elements that identify the "what to do" aspects of a service development scheme. The model enables practitioners to craft aproject planand to identify the milestones of a service-oriented initiative. SOMF also provides a common modeling notation to address alignment between business and IT organizations. Someenterprise architectsbelieve that SOA can help businesses respond more quickly and more cost-effectively to changing market conditions.[28]This style ofarchitecturepromotes reuse at the macro (service) level rather than micro (classes) level. It can also simplify interconnection to—and usage of—existing IT (legacy) assets. With SOA, the idea is that an organization can look at a problem holistically. A business has more overall control. Theoretically there would not be a mass of developers using whatever tool sets might please them. But rather they would be coding to a standard that is set within the business. They can also develop enterprise-wide SOA that encapsulates a business-oriented infrastructure. SOA has also been illustrated as a highway system providing efficiency for car drivers. The point being that if everyone had a car, but there was no highway anywhere, things would be limited and disorganized, in any attempt to get anywhere quickly or efficiently. IBM Vice President of Web Services Michael Liebow says that SOA "builds highways".[29] In some respects, SOA could be regarded as an architectural evolution rather than as a revolution. It captures many of thebest practicesof previous software architectures. In communications systems, for example, little development of solutions that use truly static bindings to talk to other equipment in the network has taken place. By embracing a SOA approach, such systems can position themselves to stress the importance of well-defined, highly inter-operable interfaces. Other predecessors of SOA includeComponent-based software engineeringand Object-Oriented Analysis and Design (OOAD) of remote objects, for instance, inCORBA. A service comprises a stand-alone unit of functionality available only via a formally defined interface. Services can be some kind of "nano-enterprises" that are easy to produce and improve. Also services can be "mega-corporations" constructed as the coordinated work of subordinate services. Reasons for treating the implementation of services as separate projects from larger projects include: SOA promises to simplify testing indirectly. Services are autonomous, stateless, with fully documented interfaces, and separate from the cross-cutting concerns of the implementation. If an organization possesses appropriately defined test data, then a corresponding stub is built that reacts to the test data when a service is being built. A full set of regression tests, scripts, data, and responses is also captured for the service. The service can be tested as a 'black box' using existing stubs corresponding to the services it calls. Test environments can be constructed where the primitive and out-of-scope services are stubs, while the remainder of the mesh is test deployments of full services. As each interface is fully documented with its own full set of regression test documentation, it becomes simple to identify problems in test services. Testing evolves to merely validate that the test service operates according to its documentation, and finds gaps in documentation and test cases of all services within the environment. Managing the data state ofidempotentservices is the only complexity. Examples may prove useful to aid in documenting a service to the level where it becomes useful. The documentation of some APIs within the Java Community Process provide good examples. As these are exhaustive, staff would typically use only important subsets. The 'ossjsa.pdf' file withinJSR-89exemplifies such a file.[31] SOA has been conflated withWeb services;[32]however, Web services are only one option to implement the patterns that comprise the SOA style. In the absence of native or binary forms of remote procedure call (RPC), applications could run more slowly and require more processing power, increasing costs. Most implementations do incur these overheads, but SOA can be implemented using technologies (for example,Java Business Integration(JBI),Windows Communication Foundation(WCF) anddata distribution service(DDS)) that do not depend on remote procedure calls or translation through XML or JSON. At the same time, emerging open-source XML parsing technologies (such asVTD-XML) and various XML-compatible binary formats promise to significantly improve SOA performance.[33][34][35] Stateful services require both the consumer and the provider to share the same consumer-specific context, which is either included in or referenced by messages exchanged between the provider and the consumer. This constraint has the drawback that it could reduce the overallscalabilityof the service provider if the service-provider needs to retain the shared context for each consumer. It also increases the coupling between a service provider and a consumer and makes switching service providers more difficult.[36]Ultimately, some critics feel that SOA services are still too constrained by applications they represent.[37] A primary challenge faced by service-oriented architecture is managing of metadata. Environments based on SOA include many services which communicate among each other to perform tasks. Due to the fact that the design may involve multiple services working in conjunction, an Application may generate millions of messages. Further services may belong to different organizations or even competing firms creating a huge trust issue. Thus SOA governance comes into the scheme of things.[38] Another major problem faced by SOA is the lack of a uniform testing framework. There are no tools that provide the required features for testing these services in a service-oriented architecture. The major causes of difficulty are:[39] Application programming interfaces (APIs) are the frameworks through which developers can interact with a web application. Tim O'Reillycoined the term "Web 2.0" to describe a perceived, quickly growing set of web-based applications.[40]A topic that has experienced extensive coverage involves the relationship between Web 2.0 and service-oriented architectures.[which?] SOA is the philosophy of encapsulating application logic in services with a uniformly defined interface and making these publicly available via discovery mechanisms. The notion of complexity-hiding and reuse, but also the concept of loosely coupling services has inspired researchers to elaborate on similarities between the two philosophies, SOA and Web 2.0, and their respective applications. Some argue Web 2.0 and SOA have significantly different elements and thus can not be regarded "parallel philosophies", whereas others consider the two concepts as complementary and regard Web 2.0 as the global SOA.[41] The philosophies of Web 2.0 and SOA serve different user needs and thus expose differences with respect to the design and also the technologies used in real-world applications. However, as of 2008[update], use-cases demonstrated the potential of combining technologies and principles of both Web 2.0 and SOA.[41] Microservices are a modern interpretation of service-oriented architectures used to builddistributed software systems. Services in a microservice architecture[42]areprocessesthat communicate with each other over thenetworkin order to fulfill a goal. These services use technology agnosticprotocols,[43]which aid in encapsulating choice of language and frameworks, making their choice a concern internal to the service. Microservices are a new realisation and implementation approach to SOA, which have become popular since 2014 (and after the introduction ofDevOps), and which also emphasize continuous deployment and other agile practices.[44] There is no single commonly agreed definition of microservices. The following characteristics and principles can be found in the literature: Interactive applications requiring real-time response times, for example low-latency interactive 3d applications, are using specific service oriented architectures addressing the specific needs of such kind of applications. These include for example low-latency optimized distributed computation and communication as well as resource and instance management.[45][46][47]	https://en.wikipedia.org/wiki/Service-oriented_architecture
Aspace-based architecture(SBA) is an approach to distributed computing systems where the various components interact with each other byexchangingtuples or entries via one or more shared spaces. This is contrasted with the more commonmessage queuing serviceapproaches where the various components interact with each other byexchangingmessages via a message broker. In a sense, both approaches exchange messages with some central agent, but how they exchange messages is very distinctive. An analogy might be where a message broker is like anacademic conference, where each presenter has the stage, and presents in the order they are scheduled; whereas a tuple space is like anunconference, where all participants can write on a common whiteboard concurrently, and all can see it at the same time. A key goal of both approaches is to createloosely-coupledsystems that minimize configuration, especially shared knowledge of who does what, leading to the objectives of better availability, resilience, scalability, etc. More specifically, an SBA is adistributed-computingarchitecture for achieving linearscalabilityof stateful, high-performance applications using thetuple spaceparadigm. It follows many of the principles ofrepresentational state transfer(REST),service-oriented architecture(SOA) andevent-driven architecture(EDA), as well as elements ofgrid computing. With a space-based architecture, applications are built out of a set of self-sufficient units, known as processing-units (PU). These units are independent of each other, so that the application can scale by adding more units. The SBA model is closely related to other patterns that have been proved successful in addressing the application scalability challenge, such asshared nothing architecture(SN), used by Google, Amazon.com and other well-known companies. The model has also been applied by many firms in the securities industry for implementing scalable electronic securities trading applications. Space-based architecture (SBA) was originally invented and developed at Microsoft in 1997–98. Internally at Microsoft it was known as Youkon Distributed Caching platform (YDC). The first large web projects based on it were MSN Live Search (released in Sept. 1999) and later MSN customer marketing data store (multi-terabyte in-memory DB shared by all MSN sites) as well as a number of other MSN sites released in late 1990s and early 2000s. See US patents 6,453,404 and 6,449,695:[1][2]and other patents based on these:[3] An application built on the principles of space-based architecture typically has the following components: Articles/papers, technical:	https://en.wikipedia.org/wiki/Space-based_architecture
A softwarecode auditis a comprehensive analysis ofsource codein aprogrammingproject with the intent of discovering bugs, security breaches or violations of programming conventions. It is an integral part of thedefensive programmingparadigm, which attempts to reduce errors before the software is released. When auditing software, every critical component should be audited separately and together with the entire program. It is a good idea to search for high-riskvulnerabilitiesfirst and work down to low-risk vulnerabilities. Vulnerabilities in between high-risk and low-risk generally exist depending on the situation and how the source code in question is being used. Application penetration testing tries to identify vulnerabilities in software by launching as many known attack techniques as possible on likely access points in an attempt to bring down the application.[1]This is a common auditing method and can be used to find out if any specific vulnerabilities exist, but not where they are in the source code. Some claim that end-of-cycle audit methods tend to overwhelm developers, ultimately leaving the team with a long list of known problems, but little actual improvement; in these cases, an in-line auditing approach is recommended as an alternative. An example of a proactive approach is the free code audit service offered by GooApps, which aims to identify and mitigate vulnerabilities early in the development process to ensure the success of mobile applications.[2] Some common high-risk vulnerabilities may exist due to the use of: The following is a list of low-risk vulnerabilities that should be found when auditing code, but do not produce a high risk situation. Source code auditing tools generally look for common vulnerabilities and only work for specificprogramming languages. Such automated tools could be used to save time, but should not be relied on for an in-depth audit. Applying such tools as part of a policy-based approach is recommended.[4] If set to the low threshold, most of the software auditing tools detect a lot of vulnerabilities, especially if the code has not been audited before. However the actual importance of these alerts also depends on how the application is used. The library that may be linked with the malicious code (and must be immune against it) has very strict requirements like cloning all returned data structures, as theintentionalattempts to break the system are expected. The program that may only be exposed to the malicious input (like web server backend) must first care about this input (buffer overruns, SQL injection, etc.). Such attacks may never occur for the program that is only internally used by authorized users in a protected infrastructure.	https://en.wikipedia.org/wiki/Code_audit
Insoftware development, adocumentation generatoris anautomationtechnology that generatesdocumentation. A generator is often used to generateAPI documentationwhich is generally forprogrammersor operational documents (such as a manual) forend users. A generator often pulls content fromsource,binaryorlogfiles.[1]Some generators, such asJavadocandDoxygen, use specialsource code commentsto drive content and formatting. This article related to a type ofsoftwareis astub. You can help Wikipedia byexpanding it.	https://en.wikipedia.org/wiki/Documentation_generator
Inprogramming language theory,semanticsis the rigorous mathematical study of the meaning ofprogramming languages.[1]Semantics assignscomputationalmeaning to validstringsin aprogramming language syntax. It is closely related to, and often crosses over with, thesemantics of mathematical proofs. Semanticsdescribes the processes a computer follows whenexecutinga program in that specific language. This can be done by describing the relationship between the input and output of a program, or giving an explanation of how the program will be executed on a certainplatform, thereby creating amodel of computation. In 1967,Robert W. Floydpublished the paperAssigning meanings to programs; his chief aim was "a rigorous standard for proofs about computer programs, includingproofs of correctness, equivalence, and termination".[2][3]Floyd further wrote:[2] A semantic definition of a programming language, in our approach, is founded on asyntacticdefinition. It must specify which of the phrases in a syntactically correct program representcommands, and whatconditionsmust be imposed on an interpretation in the neighborhood of each command. In 1969,Tony Hoarepublished a paper onHoare logicseeded by Floyd's ideas, now sometimes collectively calledaxiomatic semantics.[4][5] In the 1970s, the termsoperational semanticsanddenotational semanticsemerged.[5] The field of formal semantics encompasses all of the following: It has close links with other areas ofcomputer sciencesuch asprogramming language design,type theory,compilersandinterpreters,program verificationandmodel checking. There are many approaches to formal semantics; these belong to three major classes: Apart from the choice between denotational, operational, or axiomatic approaches, most variations in formal semantic systems arise from the choice of supporting mathematical formalism.[citation needed] Some variations of formal semantics include the following: For a variety of reasons, one might wish to describe the relationships between different formal semantics. For example: It is also possible to relate multiple semantics throughabstractionsvia the theory ofabstract interpretation.[citation needed]	https://en.wikipedia.org/wiki/Formal_semantics_of_programming_languages
FX-87is a polymorphic typed functional language based on a system forstatic program analysisin which every expression has two static properties: a type and an effect.[1]In a study done by MIT, FX-87 yields similar performance results as functional languages on programs that do not containside effects(Fibonacci,Factorial). FX-87 did yield a great performance increase when matchingDNAsequences.[2] KFXis thekernellanguage of FX-87. It was described in 'Polymorphic Effect Systems', J.M. Lucassen et al.,Proceedings of the 15th Annual ACM Conference POPL, ACM 1988, pp. 47–57. Thisprogramming-language-related article is astub. You can help Wikipedia byexpanding it.	https://en.wikipedia.org/wiki/FX-87
ISO 26262, titled "Road vehicles – Functional safety", is an international standard forfunctional safetyof electrical and/or electronic systems that are installed in serial production road vehicles (excluding mopeds), defined by theInternational Organization for Standardization(ISO) in 2011, and revised in 2018. Functional safety features form an integral part of eachautomotiveproduct development phase, ranging from the specification, to design, implementation, integration, verification, validation, and production release. The standard ISO 26262 is an adaptation of the Functional Safety standardIEC 61508for Automotive Electric/Electronic Systems. ISO 26262 defines functional safety for automotive equipment applicable throughout the lifecycle of all automotive electronic and electrical safety-related systems. The first edition (ISO 26262:2011), published on 11 November 2011, was limited to electrical and/or electronic systems installed in "series production passengercars" with a maximum gross weight of 3,500 kilograms (7,700 lb). The second edition (ISO 26262:2018), published in December 2018, extended the scope from passenger cars to all roadvehiclesexceptmopeds.[1] The standard aims to address possible hazards caused by the malfunctioning behaviour of electronic and electrical systems in vehicles. Although entitled "Road vehicles – Functional safety" the standard relates to the functional safety of Electrical and Electronic systems as well as that of systems as a whole or of their mechanical subsystems. Like its parent standard,IEC 61508, ISO 26262 is a risk-based safety standard, where the risk of hazardous operational situations is qualitatively assessed and safety measures are defined to avoid or control systematic failures and to detect or control random hardware failures, or mitigate their effects. Goals of ISO 26262: ISO 26262:2018 consists of twelve parts, ten normative parts (parts 1 to 9 and 12) and two guidelines (parts 10 and 11):[citation needed] In comparison, ISO 26262:2011 consisted of just 10 parts, with slightly different naming: ISO 26262 specifies a vocabulary (aProject Glossary) of terms, definitions, and abbreviations for application in all parts of the standard.[1]Of particular importance is the careful definition offault,error, andfailureas these terms are key to the standard’s definitions of functional safety processes,[3]particularly in the consideration that "Afaultcan manifest itself as anerror... and theerrorcan ultimately cause afailure".[1]A resultingmalfunctionthat has ahazardouseffect represents a loss offunctional safety. Note:In contrast to otherFunctional Safetystandards and the updated ISO 26262:2018,Fault Tolerancewas not explicitly defined in ISO 26262:2011 – since it was assumed impossible to comprehend all possible faults in a system.[4] Note:ISO 26262 does not use theIEC 61508termSafe failure fraction(SFF). The termssingle point faults metricandlatent faults metricare used instead.[5] ISO 26262 provides a standard forfunctional safetymanagement for automotive applications, defining standards for overall organizational safety management as well as standards for asafety life cyclefor the development and production of individual automotive products.[6][7][8][9]The ISO 26262 safety life cycle described in the next section operates on the following safety management concepts:[1] Processes within the ISO 26262safety life cycleidentify and assess hazards (safety risks), establish specific safety requirements to reduce those risks to acceptable levels, and manage and track those safety requirements to produce reasonable assurance that they are accomplished in the delivered product. These safety-relevant processes may be viewed as being integrated or running in parallel with a managed requirements life cycle of a conventionalQuality Management System:[10][11] ISO 26262 defines objectives for integral processes that are supportive to the Safety Life Cycle processes, but are continuously active throughout all phases, and also defines additional considerations that support accomplishment of general process objectives. Automotive Safety Integrity Levelrefers to an abstract classification of inherent safety risk in an automotive system or elements of such a system. ASIL classifications are used within ISO 26262 to express the level of risk reduction required to prevent a specific hazard, with ASIL D representing the highest hazard level and ASIL A the lowest. The ASIL assessed for a given hazard is then assigned to the safety goal set to address that hazard and is then inherited by the safety requirements derived from that goal.[12] The determination of ASIL is the result ofhazard analysis and risk assessment.[13]In the context of ISO 26262, a hazard is assessed based on the relative impact of hazardous effects related to a system, as adjusted for relative likelihoods of the hazard manifesting those effects. That is, each hazardous event is assessed in terms of severity of possible injuries within the context of the relative amount of time a vehicle is exposed to the possibility of the hazard happening as well as the relative likelihood that a typical driver can act to prevent the injury.[14] At the beginning of thesafety life cycle, hazard analysis and risk assessment is performed, resulting in assessment of ASIL to all identified hazardous events and safety goals. Eachhazardous eventis classified according to theseverity(S) ofinjuriesit can be expected to cause: Risk Managementrecognizes that consideration of the severity of a possible injury is modified by how likely the injury is to happen; that is, for a given hazard, a hazardous event is considered a lower risk if it is less likely to happen. Within thehazard analysis and risk assessmentprocess of this standard, the likelihood of an injurious hazard is further classified according to a combination of In terms of these classifications, anAutomotive Safety Integrity Level Dhazardous event (abbreviatedASIL D) is defined as an event having reasonable possibility of causing a life-threatening (survival uncertain) or fatal injury, with the injury being physically possible in most operating conditions, and with little chance the driver can do something to prevent the injury. That is,ASIL Dis the combination of S3, E4, and C3 classifications. For each single reduction in any one of these classifications from its maximum value (excluding reduction of C1 to C0), there is a single-level reduction in the ASIL fromD.[15][For example, a hypothetical uncontrollable (C3) fatal injury (S3) hazard could be classified asASIL Aif the hazard has a very low probability (E1).] The ASIL level belowAis the lowest level,QM.QMrefers to the standard's consideration that belowASIL A; there is no safety relevance and only standard Quality Management processes are required.[13] These Severity, Exposure, and Control definitions are informative, not prescriptive, and effectively leave some room for subjective variation or discretion between various automakers and component suppliers.[14][16]In response, theSociety for Automotive Safety Engineers (SAE)has issuedJ2980 – Considerations for ISO26262 ASIL Hazard Classificationto provide more explicit guidance for assessing Exposure, Severity and Controllability for a given hazard.[17]	https://en.wikipedia.org/wiki/ISO_26262
ISO/IEC 9126Software engineering — Product qualitywas aninternational standardfor theevaluationofsoftware quality. It has been replaced byISO/IEC 25010:2011.[1] The fundamental objective of the ISO/IEC 9126 standard is to address some of the well-known human biases that can adversely affect the delivery and perception of a software development project. These biases include changing priorities after the start of a project or not having any clear definitions of "success". By clarifying, then agreeing on the project priorities and subsequently converting abstract priorities (compliance) to measurable values (output data can be validated against schema X with zero intervention), ISO/IEC 9126 tries to develop a common understanding of the project's objectives and goals. The standard is divided into four parts: The quality model presented in the first part of the standard, ISO/IEC 9126-1,[2]classifiessoftware qualityin a structured set of characteristics and sub-characteristics as follows: Each quality sub-characteristic (e.g. adaptability) is further divided into attributes. An attribute is an entity which can be verified or measured in the software product. Attributes are not defined in the standard, as they vary between different software products. Software product is defined in a broad sense: it encompasses executables, source code, architecture descriptions, and so on. As a result, the notion of user extends to operators as well as to programmers, which are users of components such as software libraries. The standard provides a framework for organizations to define a quality model for a software product. On doing so, however, it leaves up to each organization the task of specifying precisely its own model. This may be done, for example, by specifying target values for quality metrics which evaluates the degree of presence of quality attributes. Internal metrics are those which do not rely on software execution (static measure). External metrics are applicable to running software. Quality-in-use metrics are only available when the final product is used in real conditions. Ideally, the internal quality determines the external quality and external quality determines quality in use. This standard stems from the GE model for describing software quality, presented in 1977 by McCall et al., which is organized around three types of quality characteristic: ISO/IEC 9126 distinguishes between a defect and a nonconformity, adefectbeing "The nonfulfilment of intended usage requirements", whereas anonconformityis "The nonfulfilment of specified requirements". A similar distinction is made between validation and verification, known as V&V in the testing trade. ISO/IEC 9126 was issued on December 19, 1991. On June 15, 2001, ISO/IEC 9126:1991 was replaced by ISO/IEC 9126:2001 (four parts 9126–1 to 9126–4). On March 1, 2011, ISO/IEC 9126 was replaced by ISO/IEC25010:2011 Systems and software engineering - Systems and software Quality Requirements and Evaluation (SQuaRE) - System and software quality models. Compared to 9126, "security" and "compatibility" were added as main characteristics. ISO/IEC then started work onSQuaRE(Software product Quality Requirements and Evaluation), a more extensive series of standards to replace ISO/IEC 9126, with numbers of the form ISO/IEC 250mn. For instance, ISO/IEC 25000 was issued in 2005, andISO/IEC 25010, which supersedes ISO/IEC 9126-1, was issued in March 2011. ISO 25010 has eight product quality characteristics (in contrast to ISO 9126's six), and 31 subcharacteristics.[3]	https://en.wikipedia.org/wiki/ISO/IEC_9126
Lintis thecomputer scienceterm for astatic code analysistool used to flag programming errors,bugs, stylistic errors and suspicious constructs.[4]The term originates from aUnixutilitythat examinedC languagesource code.[1]A program which performs this function is also known as a "linter". Stephen C. Johnson, a computer scientist atBell Labs, came up with the term "lint" in 1978 while debugging theyaccgrammar he was writing forCand dealing withportabilityissues stemming from portingUnixto a32-bitmachine.[5][1]The term was borrowed from the wordlint, the tiny bits of fiber and fluff shed by clothing, as the command he wrote would act like a lint trap in a clothes dryer, capturing waste fibers while leaving whole fabrics intact. In 1979, lint programming was used outside of Bell Labs for the first time, in the seventh version (V7) of Unix. Over the years, different versions of lint have been developed for manyCandC++compilers, and while modern-day compilers have lint-like functions, lint-like tools have also advanced their capabilities. For example, Gimpel'sPC-Lint, introduced in 1985 and used to analyze C++ source code, is still for sale.[5] In his original 1978 paper Johnson stated his reasoning in creating a separate program to detect errors, distinct from that which it analyzed: "...the general notion of having two programs is a good one" [because they concentrate on different things, thereby allowing the programmer to] "concentrate at one stage of the programming process solely on thealgorithms,data structures, and correctness of the program, and then later retrofit, with the aid of lint, the desirable properties of universality and portability".[1] The analysis performed by lint-like tools can also be performed by anoptimizing compiler, which aims to generate faster code. Even though moderncompilershave evolved to include many of lint's historical functions, lint-like tools have also evolved to detect an even wider variety of suspicious constructs. These include "warnings about syntax errors, uses ofundeclared variables, calls to deprecated functions, spacing and formatting conventions, misuse of scope, implicit fallthrough inswitch statements, missing license headers, [and]...dangerous language features".[6] Lint-like tools are especially useful fordynamically typed languageslikeJavaScriptandPython. Because the interpreters of such languages typically do not enforce as many and as strict rules duringexecution, linter tools can also be used as simpledebuggersfor finding common errors (e.g. syntactic discrepancies) as well as hard-to-find errors such asheisenbugs(drawing attention to suspicious code as "possible errors").[7]Lint-like tools generally performstatic analysisof source code.[8] Lint-like tools have also been developed for other aspects of software development, such as enforcing grammar and style guides for given language source code.[9]Some tools (such asESLint) also allow rules to be auto-fixable: a rule definition can also come with the definition of a transform that resolves the warning. Rules about style are especially likely to come with an auto-fix. If the linter is run in "fix all" mode on a file that triggers only rules about formatting, the linter will act just like a formatter.	https://en.wikipedia.org/wiki/Lint_(software)
This is a list of notable tools forstatic program analysis(program analysis is a synonym for code analysis). (7.9) (6.3.5) Tools that usesound, i.e. over-approximating a rigorous model,formal methodsapproach to static analysis (e.g., using staticprogram assertions). Sound methods contain no false negatives for bug-free programs, at least with regards to the idealized mathematical model they are based on (there is no "unconditional" soundness). Note that there is no guarantee they will reportallbugs for buggy programs, they will report at least one.	https://en.wikipedia.org/wiki/List_of_tools_for_static_code_analysis
Inprogram analysis,shape analysisis astatic code analysistechnique that discovers and verifies properties oflinked,dynamically allocateddata structures in (usuallyimperative) computer programs. It is typically used at compile time to find software bugs or to verify high-level correctness properties of programs. InJavaprograms, it can be used to ensure that a sort method correctly sorts a list. ForCprograms, it might look for places where a block of memory is not properly freed. Shape analysis has been applied to a variety of problems: Shape analysis is a form ofpointer analysis, although it is more precise than typical pointer analysis. Pointer analysis attempts to determine the set of objects to which a pointer can point (called the points-to set of the pointer). Unfortunately, these analysis are necessarily approximate (since a perfectly precise static analysis could solve thehalting problem). Shape analysis can determine smaller (more precise) points-to sets. Consider the following simpleC++program. This program builds an array of objects, processes them in some arbitrary way, and then deletes them. Assuming that theprocess_itemsfunction is free of errors, it is clear that the program is safe: it never references freed memory, and it deletes all the objects that it has constructed. Unfortunately, most pointer analyses have difficulty analyzing this program precisely. In order to determine points-to sets, a pointer analysis must be able tonamea program's objects. In general, programs can allocate an unbounded number of objects; but in order to terminate, a pointer analysis can only use a finite set of names. A typical approximation is to give all the objects allocated on a given line of the program the same name. In the example above, all the objects constructed at line [1] would have the same name. Therefore, when thedeletestatement is analyzed for the first time, the analysis determines that one of the objects named [1] is being deleted. The second time the statement is analyzed (since it is in a loop) the analysis warns of a possible error: since it is unable to distinguish the objects in the array, it may be that the seconddeleteis deleting the same object as the firstdelete. This warning is spurious, and the goal of shape analysis is to avoid such warnings. Shape analysis overcomes the problems of pointer analysis by using a more flexible naming system for objects. Rather than giving an object the same name throughout a program, objects can change names depending on the program's actions. Sometimes, several distinct objects with different names may besummarized,or merged, so that they have the same name. Then, when a summarized object is about to be used by the program, it can bematerialized—that is, the summarized object is split into two objects with distinct names, one representing a single object and the other representing the remaining summarized objects. The basic heuristic of shape analysis is that objects that are being used by the program are represented using unique materialized objects, while objects not in use are summarized. The array of objects in the example above is summarized in separate ways at lines [1], [2], and [3]. At line [1], the array has been only partly constructed. The array elements 0..i-1 contain constructed objects. The array element i is about to be constructed, and the following elements are uninitialized. Shape analysis can approximate this situation using a summary for the first set of elements, a materialized memory location for element i, and a summary for the remaining uninitialized locations, as follows: After the loop terminates, at line [2], there is no need to keep anything materialized. The shape analysis determines at this point that all the array elements have been initialized: At line [3], however, the array elementiis in use again. Therefore, the analysis splits the array into three segments as in line [1]. This time, though, the first segment beforeihas been deleted, and the remaining elements are still valid (assuming thedeletestatement hasn't executed yet). Notice that in this case, the analysis recognizes that the pointer at indexihas not been deleted yet. Therefore, it doesn't warn of a double deletion.	https://en.wikipedia.org/wiki/Shape_analysis_(program_analysis)
Software quality assurance(SQA) is a means and practice of monitoring allsoftware engineeringprocesses, methods, and work products to ensure compliance against defined standards.[1]It may include ensuring conformance to standards or models, such asISO/IEC 9126(now superseded by ISO 25010),SPICEorCMMI.[2] It includes standards and procedures that managers, administrators or developers may use to review and audit software products and activities to verify that the software meets quality criteria which link to standards. SQA encompasses the entiresoftware developmentprocess, includingrequirements engineering,software design,coding,code reviews,source code control,software configuration management,testing,release managementandsoftware integration. It is organized into goals, commitments, abilities, activities, measurements,verification and validation.[3] SQA involves a three-pronged approach: Guidelines for the application of ISO 9001:2015 to computer software are described in ISO/IEC/IEEE 90003:2018.[4]External entities can be contracted as part of process assessments to verify that projects are standard-compliant. More specifically in case of software,ISO/IEC 9126(now superseded by ISO 25010) should be considered and applied for software quality. Quality assuranceactivities take place at each phase of development. Analysts use application technology and techniques to achieve high-quality specifications and designs, such asmodel-driven design. Engineers and technicians find bugs and problems with related software quality through testing activities. Standards and process deviations are identified and addressed throughout development byproject managersor quality managers, who also ensure that changes to functionality, performance, features, architecture and component (in general:changes to product or servicescope) are made only after appropriate review, e.g. as part ofchange control boards.[5]	https://en.wikipedia.org/wiki/Software_quality_assurance
SonarQube(formerlySonar)[3]is anopen-sourceplatform developed bySonarSourcefor continuous inspection ofcode qualityto perform automatic reviews with staticanalysis of codeto detectbugsandcode smellson 29programming languages. SonarQube offers reports onduplicated code,coding standards,unit tests,code coverage,code complexity,comments,bugs, and security recommendations.[4][5] SonarQube provides automatedanalysisand integration withMaven,Ant,Gradle,MSBuild, andcontinuous integrationtools.[6][7][8] SonarQube supports the programming languages:Java(including Android),C#,C,C++,JavaScript,TypeScript,Python,Go,Swift,COBOL,Apex,PHP,Kotlin,Ruby,Scala,HTML,CSS,ABAP,Flex,Objective-C,PL/I,PL/SQL,RPG,T-SQL,VB.NET,VB6, andXML.[9]As of December 2021, analyzingC,C++,Objective-C,Swift,ABAP,T-SQL, andPL/SQLis only available via a commercial license. SonarQube is an open core product for static code analysis, with additional features offered in commercial editions. SonarQube is expandable with the use ofplug-ins. It integrates withEclipse,Visual Studio,Visual Studio Code, andIntelliJ IDEAdevelopment environments throughSonarQube for IDE[10]plug-ins, as well as external tools such asLDAP,Active Directory, andGitHub.[11][12] In 2009, SonarQube received aJolt Awardunder thetesting toolscategory.[13][14]	https://en.wikipedia.org/wiki/SonarQube
Computer security(alsocybersecurity,digital security, orinformation technology (IT) security) is a subdiscipline within the field ofinformation security. It consists of the protection ofcomputer software,systemsandnetworksfromthreatsthat can lead to unauthorized information disclosure, theft or damage tohardware,software, ordata, as well as from the disruption or misdirection of theservicesthey provide.[1][2] The significance of the field stems from the expanded reliance oncomputer systems, theInternet,[3]andwireless network standards. Its importance is further amplified by the growth ofsmart devices, includingsmartphones,televisions, and the various devices that constitute theInternet of things(IoT). Cybersecurity has emerged as one of the most significant new challenges facing the contemporary world, due to both the complexity ofinformation systemsand the societies they support. Security is particularly crucial for systems that govern large-scale systems with far-reaching physical effects, such aspower distribution,elections, andfinance.[4][5] Although many aspects of computer security involve digital security, such as electronicpasswordsandencryption,physical securitymeasures such asmetal locksare still used to prevent unauthorized tampering. IT security is not a perfect subset ofinformation security, therefore does not completely align into thesecurity convergenceschema. A vulnerability refers to a flaw in the structure, execution, functioning, or internal oversight of a computer or system that compromises its security. Most of the vulnerabilities that have been discovered are documented in theCommon Vulnerabilities and Exposures(CVE) database.[6]Anexploitablevulnerability is one for which at least one workingattackorexploitexists.[7]Actors maliciously seeking vulnerabilities are known asthreats. Vulnerabilities can be researched, reverse-engineered, hunted, or exploited usingautomated toolsor customized scripts.[8][9] Various people or parties are vulnerable to cyber attacks; however, different groups are likely to experience different types of attacks more than others.[10] In April 2023, theUnited KingdomDepartment for Science, Innovation & Technology released a report on cyber attacks over the previous 12 months.[11]They surveyed 2,263 UK businesses, 1,174 UK registered charities, and 554 education institutions. The research found that "32% of businesses and 24% of charities overall recall any breaches or attacks from the last 12 months." These figures were much higher for "medium businesses (59%), large businesses (69%), and high-income charities with £500,000 or more in annual income (56%)."[11]Yet, although medium or large businesses are more often the victims, since larger companies have generally improved their security over the last decade,small and midsize businesses(SMBs) have also become increasingly vulnerable as they often "do not have advanced tools to defend the business."[10]SMBs are most likely to be affected by malware, ransomware, phishing,man-in-the-middle attacks, and Denial-of Service (DoS) Attacks.[10] Normal internet users are most likely to be affected by untargeted cyberattacks.[12]These are where attackers indiscriminately target as many devices, services, or users as possible. They do this using techniques that take advantage of the openness of the Internet. These strategies mostly includephishing,ransomware,water holingand scanning.[12] To secure a computer system, it is important to understand the attacks that can be made against it, and thesethreatscan typically be classified into one of the following categories: Abackdoorin a computer system, acryptosystem, or analgorithmis any secret method of bypassing normalauthenticationor security controls. These weaknesses may exist for many reasons, including original design or poor configuration.[13]Due to the nature of backdoors, they are of greater concern to companies and databases as opposed to individuals. Backdoors may be added by an authorized party to allow some legitimate access or by an attacker for malicious reasons.Criminalsoften usemalwareto install backdoors, giving them remote administrative access to a system.[14]Once they have access, cybercriminals can "modify files, steal personal information, install unwanted software, and even take control of the entire computer."[14] Backdoors can be difficult to detect, as they often remain hidden within the source code or system firmware intimate knowledge of theoperating systemof the computer. Denial-of-service attacks(DoS) are designed to make a machine or network resource unavailable to its intended users.[15]Attackers can deny service to individual victims, such as by deliberately entering a wrong password enough consecutive times to cause the victim's account to be locked, or they may overload the capabilities of a machine or network and block all users at once. While a network attack from a singleIP addresscan be blocked by adding a new firewall rule, many forms ofdistributed denial-of-service(DDoS) attacks are possible, where the attack comes from a large number of points. In this case, defending against these attacks is much more difficult. Such attacks can originate from thezombie computersof abotnetor from a range of other possible techniques, includingdistributed reflective denial-of-service(DRDoS), where innocent systems are fooled into sending traffic to the victim.[15]With such attacks, the amplification factor makes the attack easier for the attacker because they have to use little bandwidth themselves. To understand why attackers may carry out these attacks, see the 'attacker motivation' section. A direct-access attack is when an unauthorized user (an attacker) gains physical access to a computer, most likely to directly copy data from it or steal information.[16]Attackers may also compromise security by making operating system modifications, installingsoftware worms,keyloggers,covert listening devicesor using wireless microphones. Even when the system is protected by standard security measures, these may be bypassed by booting another operating system or tool from aCD-ROMor other bootable media.Disk encryptionand theTrusted Platform Modulestandard are designed to prevent these attacks. Direct service attackers are related in concept todirect memory attackswhich allow an attacker to gain direct access to a computer's memory.[17]The attacks "take advantage of a feature of modern computers that allows certain devices, such as external hard drives, graphics cards, or network cards, to access the computer's memory directly."[17] Eavesdroppingis the act of surreptitiously listening to a private computer conversation (communication), usually between hosts on a network. It typically occurs when a user connects to a network where traffic is not secured or encrypted and sends sensitive business data to a colleague, which, when listened to by an attacker, could be exploited.[18]Data transmitted across anopen networkallows an attacker to exploit a vulnerability and intercept it via various methods. Unlikemalware, direct-access attacks, or other forms of cyber attacks, eavesdropping attacks are unlikely to negatively affect the performance of networks or devices, making them difficult to notice.[18]In fact, "the attacker does not need to have any ongoing connection to the software at all. The attacker can insert the software onto a compromised device, perhaps by direct insertion or perhaps by a virus or other malware, and then come back some time later to retrieve any data that is found or trigger the software to send the data at some determined time."[19] Using avirtual private network(VPN), which encrypts data between two points, is one of the most common forms of protection against eavesdropping. Using the best form of encryption possible for wireless networks is best practice, as well as usingHTTPSinstead of an unencryptedHTTP.[20] Programs such asCarnivoreandNarusInSighthave been used by theFederal Bureau of Investigation(FBI) and NSA to eavesdrop on the systems ofinternet service providers. Even machines that operate as a closed system (i.e., with no contact with the outside world) can be eavesdropped upon by monitoring the faintelectromagnetictransmissions generated by the hardware.TEMPESTis a specification by the NSA referring to these attacks. Malicious software (malware) is any software code or computer program "intentionally written to harm a computer system or its users."[21]Once present on a computer, it can leak sensitive details such as personal information, business information and passwords, can give control of the system to the attacker, and can corrupt or delete data permanently.[22][23] Man-in-the-middle attacks(MITM) involve a malicious attacker trying to intercept, surveil or modify communications between two parties by spoofing one or both party's identities and injecting themselves in-between.[24]Types of MITM attacks include: Surfacing in 2017, a new class of multi-vector,[25]polymorphic[26]cyber threats combine several types of attacks and change form to avoid cybersecurity controls as they spread. Multi-vector polymorphic attacks, as the name describes, are both multi-vectored and polymorphic.[27]Firstly, they are a singular attack that involves multiple methods of attack. In this sense, they are "multi-vectored (i.e. the attack can use multiple means of propagation such as via the Web, email and applications." However, they are also multi-staged, meaning that "they can infiltrate networks and move laterally inside the network."[27]The attacks can be polymorphic, meaning that the cyberattacks used such as viruses, worms or trojans "constantly change ("morph") making it nearly impossible to detect them using signature-based defences."[27] Phishingis the attempt of acquiring sensitive information such as usernames, passwords, and credit card details directly from users by deceiving the users.[28]Phishing is typically carried out byemail spoofing,instant messaging,text message, or on aphonecall. They often direct users to enter details at a fake website whoselook and feelare almost identical to the legitimate one.[29]The fake website often asks for personal information, such as login details and passwords. This information can then be used to gain access to the individual's real account on the real website. Preying on a victim's trust, phishing can be classified as a form ofsocial engineering. Attackers can use creative ways to gain access to real accounts. A common scam is for attackers to send fake electronic invoices[30]to individuals showing that they recently purchased music, apps, or others, and instructing them to click on a link if the purchases were not authorized. A more strategic type of phishing is spear-phishing which leverages personal or organization-specific details to make the attacker appear like a trusted source. Spear-phishing attacks target specific individuals, rather than the broad net cast by phishing attempts.[31] Privilege escalationdescribes a situation where an attacker with some level of restricted access is able to, without authorization, elevate their privileges or access level.[32]For example, a standard computer user may be able to exploit avulnerabilityin the system to gain access to restricted data; or even becomerootand have full unrestricted access to a system. The severity of attacks can range from attacks simply sending an unsolicited email to aransomware attackon large amounts of data. Privilege escalation usually starts withsocial engineeringtechniques, oftenphishing.[32] Privilege escalation can be separated into two strategies, horizontal and vertical privilege escalation: Any computational system affects its environment in some form. This effect it has on its environment can range from electromagnetic radiation, to residual effect on RAM cells which as a consequence make aCold boot attackpossible, to hardware implementation faults that allow for access or guessing of other values that normally should be inaccessible. In Side-channel attack scenarios, the attacker would gather such information about a system or network to guess its internal state and as a result access the information which is assumed by the victim to be secure. The target information in a side channel can be challenging to detect due to its low amplitude when combined with other signals[33] Social engineering, in the context of computer security, aims to convince a user to disclose secrets such as passwords, card numbers, etc. or grant physical access by, for example, impersonating a senior executive, bank, a contractor, or a customer.[34]This generally involves exploiting people's trust, and relying on theircognitive biases. A common scam involves emails sent to accounting and finance department personnel, impersonating their CEO and urgently requesting some action. One of the main techniques of social engineering arephishingattacks. In early 2016, theFBIreported that suchbusiness email compromise(BEC) scams had cost US businesses more than $2 billion in about two years.[35] In May 2016, theMilwaukee BucksNBAteam was the victim of this type of cyber scam with a perpetrator impersonating the team's presidentPeter Feigin, resulting in the handover of all the team's employees' 2015W-2tax forms.[36] Spoofing is an act of pretending to be a valid entity through the falsification of data (such as an IP address or username), in order to gain access to information or resources that one is otherwise unauthorized to obtain. Spoofing is closely related tophishing.[37][38]There are several types of spoofing, including: In 2018, the cybersecurity firmTrellixpublished research on the life-threatening risk of spoofing in the healthcare industry.[40] Tamperingdescribes amalicious modificationor alteration of data. It is an intentional but unauthorized act resulting in the modification of a system, components of systems, its intended behavior, or data. So-calledEvil Maid attacksand security services planting ofsurveillancecapability into routers are examples.[41] HTMLsmuggling allows an attacker tosmugglea malicious code inside a particular HTML or web page.[42]HTMLfiles can carry payloads concealed as benign, inert data in order to defeatcontent filters. These payloads can be reconstructed on the other side of the filter.[43] When a target user opens the HTML, the malicious code is activated; the web browser thendecodesthe script, which then unleashes the malware onto the target's device.[42] Employee behavior can have a big impact oninformation securityin organizations. Cultural concepts can help different segments of the organization work effectively or work against effectiveness toward information security within an organization. Information security culture is the "...totality of patterns of behavior in an organization that contributes to the protection of information of all kinds."[44] Andersson and Reimers (2014) found that employees often do not see themselves as part of their organization's information security effort and often take actions that impede organizational changes.[45]Indeed, the Verizon Data Breach Investigations Report 2020, which examined 3,950 security breaches, discovered 30% of cybersecurity incidents involved internal actors within a company.[46]Research shows information security culture needs to be improved continuously. In "Information Security Culture from Analysis to Change", authors commented, "It's a never-ending process, a cycle of evaluation and change or maintenance." To manage the information security culture, five steps should be taken: pre-evaluation, strategic planning, operative planning, implementation, and post-evaluation.[47] In computer security, acountermeasureis an action, device, procedure or technique that reduces a threat, a vulnerability, or anattackby eliminating or preventing it, by minimizing the harm it can cause, or by discovering and reporting it so that corrective action can be taken.[48][49][50] Some common countermeasures are listed in the following sections: Security by design, or alternately secure by design, means that the software has been designed from the ground up to be secure. In this case, security is considered a main feature. The UK government's National Cyber Security Centre separates secure cyber design principles into five sections:[51] These design principles of security by design can include some of the following techniques: Security architecture can be defined as the "practice of designing computer systems to achieve security goals."[52]These goals have overlap with the principles of "security by design" explored above, including to "make initial compromise of the system difficult," and to "limit the impact of any compromise."[52]In practice, the role of a security architect would be to ensure the structure of a system reinforces the security of the system, and that new changes are safe and meet the security requirements of the organization.[53][54] Similarly, Techopedia defines security architecture as "a unified security design that addresses the necessities and potential risks involved in a certain scenario or environment. It also specifies when and where to apply security controls. The design process is generally reproducible." The key attributes of security architecture are:[55] Practicing security architecture provides the right foundation to systematically address business, IT and security concerns in an organization. A state of computer security is the conceptual ideal, attained by the use of three processes: threat prevention, detection, and response. These processes are based on various policies and system components, which include the following: Today, computer security consists mainly of preventive measures, likefirewallsor anexit procedure. A firewall can be defined as a way of filtering network data between a host or a network and another network, such as theInternet. They can be implemented as software running on the machine, hooking into thenetwork stack(or, in the case of mostUNIX-based operating systems such asLinux, built into the operating systemkernel) to provide real-time filtering and blocking.[56]Another implementation is a so-calledphysical firewall, which consists of a separate machine filtering network traffic. Firewalls are common amongst machines that are permanently connected to the Internet. Some organizations are turning tobig dataplatforms, such asApache Hadoop, to extend data accessibility andmachine learningto detectadvanced persistent threats.[58] In order to ensure adequate security, the confidentiality, integrity and availability of a network, better known as the CIA triad, must be protected and is considered the foundation to information security.[59]To achieve those objectives, administrative, physical and technical security measures should be employed. The amount of security afforded to an asset can only be determined when its value is known.[60] Vulnerability management is the cycle of identifying, fixing or mitigatingvulnerabilities,[61]especially in software andfirmware. Vulnerability management is integral to computer security andnetwork security. Vulnerabilities can be discovered with avulnerability scanner, which analyzes a computer system in search of known vulnerabilities,[62]such asopen ports, insecure software configuration, and susceptibility tomalware. In order for these tools to be effective, they must be kept up to date with every new update the vendor release. Typically, these updates will scan for the new vulnerabilities that were introduced recently. Beyond vulnerability scanning, many organizations contract outside security auditors to run regularpenetration testsagainst their systems to identify vulnerabilities. In some sectors, this is a contractual requirement.[63] The act of assessing and reducing vulnerabilities to cyber attacks is commonly referred to asinformation technology security assessments. They aim to assess systems for risk and to predict and test for their vulnerabilities. Whileformal verificationof the correctness of computer systems is possible,[64][65]it is not yet common. Operating systems formally verified includeseL4,[66]andSYSGO'sPikeOS[67][68]– but these make up a very small percentage of the market. It is possible to reduce an attacker's chances by keeping systems up to date with security patches and updates and by hiring people with expertise in security. Large companies with significant threats can hire Security Operations Centre (SOC) Analysts. These are specialists in cyber defences, with their role ranging from "conducting threat analysis to investigating reports of any new issues and preparing and testing disaster recovery plans."[69] Whilst no measures can completely guarantee the prevention of an attack, these measures can help mitigate the damage of possible attacks. The effects of data loss/damage can be also reduced by carefulbacking upandinsurance. Outside of formal assessments, there are various methods of reducing vulnerabilities.Two factor authenticationis a method for mitigating unauthorized access to a system or sensitive information.[70]It requiressomething you know:a password or PIN, andsomething you have: a card, dongle, cellphone, or another piece of hardware. This increases security as an unauthorized person needs both of these to gain access. Protecting against social engineering and direct computer access (physical) attacks can only happen by non-computer means, which can be difficult to enforce, relative to the sensitivity of the information. Training is often involved to help mitigate this risk by improving people's knowledge of how to protect themselves and by increasing people's awareness of threats.[71]However, even in highly disciplined environments (e.g. military organizations), social engineering attacks can still be difficult to foresee and prevent. Inoculation, derived frominoculation theory, seeks to prevent social engineering and other fraudulent tricks and traps by instilling a resistance to persuasion attempts through exposure to similar or related attempts.[72] Hardware-based or assisted computer security also offers an alternative to software-only computer security. Using devices and methods such asdongles,trusted platform modules, intrusion-aware cases, drive locks, disabling USB ports, and mobile-enabled access may be considered more secure due to the physical access (or sophisticated backdoor access) required in order to be compromised. Each of these is covered in more detail below. One use of the termcomputer securityrefers to technology that is used to implementsecure operating systems. Using secure operating systems is a good way of ensuring computer security. These are systems that have achieved certification from an external security-auditing organization, the most popular evaluations areCommon Criteria(CC).[86] In software engineering,secure codingaims to guard against the accidental introduction of security vulnerabilities. It is also possible to create software designed from the ground up to be secure. Such systems aresecure by design. Beyond this, formal verification aims to prove thecorrectnessof thealgorithmsunderlying a system;[87]important forcryptographic protocolsfor example. Within computer systems, two of the mainsecurity modelscapable of enforcing privilege separation areaccess control lists(ACLs) androle-based access control(RBAC). Anaccess-control list(ACL), with respect to a computer file system, is a list of permissions associated with an object. An ACL specifies which users or system processes are granted access to objects, as well as what operations are allowed on given objects. Role-based access control is an approach to restricting system access to authorized users,[88][89][90]used by the majority of enterprises with more than 500 employees,[91]and can implementmandatory access control(MAC) ordiscretionary access control(DAC). A further approach,capability-based securityhas been mostly restricted to research operating systems. Capabilities can, however, also be implemented at the language level, leading to a style of programming that is essentially a refinement of standard object-oriented design. An open-source project in the area is theE language. The end-user is widely recognized as the weakest link in the security chain[92]and it is estimated that more than 90% of security incidents and breaches involve some kind of human error.[93][94]Among the most commonly recorded forms of errors and misjudgment are poor password management, sending emails containing sensitive data and attachments to the wrong recipient, the inability to recognize misleading URLs and to identify fake websites and dangerous email attachments. A common mistake that users make is saving their user id/password in their browsers to make it easier to log in to banking sites. This is a gift to attackers who have obtained access to a machine by some means. The risk may be mitigated by the use of two-factor authentication.[95] As the human component of cyber risk is particularly relevant in determining the global cyber risk[96]an organization is facing, security awareness training, at all levels, not only provides formal compliance with regulatory and industry mandates but is considered essential[97]in reducing cyber risk and protecting individuals and companies from the great majority of cyber threats. The focus on the end-user represents a profound cultural change for many security practitioners, who have traditionally approached cybersecurity exclusively from a technical perspective, and moves along the lines suggested by major security centers[98]to develop a culture of cyber awareness within the organization, recognizing that a security-aware user provides an important line of defense against cyber attacks. Related to end-user training,digital hygieneorcyber hygieneis a fundamental principle relating to information security and, as the analogy withpersonal hygieneshows, is the equivalent of establishing simple routine measures to minimize the risks from cyber threats. The assumption is that good cyber hygiene practices can give networked users another layer of protection, reducing the risk that one vulnerable node will be used to either mount attacks or compromise another node or network, especially from common cyberattacks.[99]Cyber hygiene should also not be mistaken forproactive cyber defence, a military term.[100] The most common acts of digital hygiene can include updating malware protection, cloud back-ups, passwords, and ensuring restricted admin rights and network firewalls.[101]As opposed to a purely technology-based defense against threats, cyber hygiene mostly regards routine measures that are technically simple to implement and mostly dependent on discipline[102]or education.[103]It can be thought of as an abstract list of tips or measures that have been demonstrated as having a positive effect on personal or collective digital security. As such, these measures can be performed by laypeople, not just security experts. Cyber hygiene relates to personal hygiene as computer viruses relate to biological viruses (or pathogens). However, while the termcomputer viruswas coined almost simultaneously with the creation of the first working computer viruses,[104]the termcyber hygieneis a much later invention, perhaps as late as 2000[105]by Internet pioneerVint Cerf. It has since been adopted by theCongress[106]andSenateof the United States,[107]the FBI,[108]EUinstitutions[99]and heads of state.[100] Responding to attemptedsecurity breachesis often very difficult for a variety of reasons, including: Where an attack succeeds and a breach occurs, many jurisdictions now have in place mandatorysecurity breach notification laws. The growth in the number of computer systems and the increasing reliance upon them by individuals, businesses, industries, and governments means that there are an increasing number of systems at risk. The computer systems of financial regulators and financial institutions like theU.S. Securities and Exchange Commission, SWIFT, investment banks, and commercial banks are prominent hacking targets forcybercriminalsinterested in manipulating markets and making illicit gains.[109]Websites and apps that accept or storecredit card numbers, brokerage accounts, andbank accountinformation are also prominent hacking targets, because of the potential for immediate financial gain from transferring money, making purchases, or selling the information on theblack market.[110]In-store payment systems andATMshave also been tampered with in order to gather customer account data andPINs. TheUCLAInternet Report: Surveying the Digital Future (2000) found that the privacy of personal data created barriers to online sales and that more than nine out of 10 internet users were somewhat or very concerned aboutcredit cardsecurity.[111] The most common web technologies for improving security between browsers and websites are named SSL (Secure Sockets Layer), and its successor TLS (Transport Layer Security),identity managementandauthenticationservices, anddomain nameservices allow companies and consumers to engage in secure communications and commerce. Several versions of SSL and TLS are commonly used today in applications such as web browsing, e-mail, internet faxing,instant messaging, andVoIP(voice-over-IP). There are variousinteroperableimplementations of these technologies, including at least one implementation that isopen source. Open source allows anyone to view the application'ssource code, and look for and report vulnerabilities. The credit card companiesVisaandMasterCardcooperated to develop the secureEMVchip which is embedded in credit cards. Further developments include theChip Authentication Programwhere banks give customers hand-held card readers to perform online secure transactions. Other developments in this arena include the development of technology such as Instant Issuance which has enabled shoppingmall kiosksacting on behalf of banks to issue on-the-spot credit cards to interested customers. Computers control functions at many utilities, including coordination oftelecommunications, thepower grid,nuclear power plants, and valve opening and closing in water and gas networks. The Internet is a potential attack vector for such machines if connected, but theStuxnetworm demonstrated that even equipment controlled by computers not connected to the Internet can be vulnerable. In 2014, theComputer Emergency Readiness Team, a division of theDepartment of Homeland Security, investigated 79 hacking incidents at energy companies.[112] Theaviationindustry is very reliant on a series of complex systems which could be attacked.[113]A simple power outage at one airport can cause repercussions worldwide,[114]much of the system relies on radio transmissions which could be disrupted,[115]and controlling aircraft over oceans is especially dangerous because radar surveillance only extends 175 to 225 miles offshore.[116]There is also potential for attack from within an aircraft.[117] Implementing fixes in aerospace systems poses a unique challenge because efficient air transportation is heavily affected by weight and volume. Improving security by adding physical devices to airplanes could increase their unloaded weight, and could potentially reduce cargo or passenger capacity.[118] In Europe, with the (Pan-European Network Service)[119]and NewPENS,[120]and in the US with the NextGen program,[121]air navigation service providersare moving to create their own dedicated networks. Many modern passports are nowbiometric passports, containing an embeddedmicrochipthat stores a digitized photograph and personal information such as name, gender, and date of birth. In addition, more countries[which?]are introducingfacial recognition technologyto reduceidentity-related fraud. The introduction of the ePassport has assisted border officials in verifying the identity of the passport holder, thus allowing for quick passenger processing.[122]Plans are under way in the US, theUK, andAustraliato introduce SmartGate kiosks with both retina andfingerprint recognitiontechnology.[123]The airline industry is moving from the use of traditional paper tickets towards the use ofelectronic tickets(e-tickets). These have been made possible by advances in online credit card transactions in partnership with the airlines. Long-distance bus companies[which?]are also switching over to e-ticketing transactions today. The consequences of a successful attack range from loss of confidentiality to loss of system integrity,air traffic controloutages, loss of aircraft, and even loss of life. Desktop computers and laptops are commonly targeted to gather passwords or financial account information or to construct a botnet to attack another target.Smartphones,tablet computers,smart watches, and othermobile devicessuch asquantified selfdevices likeactivity trackershave sensors such as cameras, microphones, GPS receivers, compasses, andaccelerometerswhich could be exploited, and may collect personal information, including sensitive health information. WiFi, Bluetooth, and cell phone networks on any of these devices could be used as attack vectors, and sensors might be remotely activated after a successful breach.[124] The increasing number ofhome automationdevices such as theNest thermostatare also potential targets.[124] Today many healthcare providers andhealth insurancecompanies use the internet to provide enhanced products and services. Examples are the use oftele-healthto potentially offer better quality and access to healthcare, or fitness trackers to lower insurance premiums.[citation needed]Patient records are increasingly being placed on secure in-house networks, alleviating the need for extra storage space.[125] Large corporations are common targets. In many cases attacks are aimed at financial gain throughidentity theftand involvedata breaches. Examples include the loss of millions of clients' credit card and financial details byHome Depot,[126]Staples,[127]Target Corporation,[128]andEquifax.[129] Medical records have been targeted in general identify theft, health insurance fraud, and impersonating patients to obtain prescription drugs for recreational purposes or resale.[130]Although cyber threats continue to increase, 62% of all organizations did not increase security training for their business in 2015.[131] Not all attacks are financially motivated, however: security firmHBGary Federalhad a serious series of attacks in 2011 fromhacktivistgroupAnonymousin retaliation for the firm's CEO claiming to have infiltrated their group,[132][133]andSony Pictureswashacked in 2014with the apparent dual motive of embarrassing the company through data leaks and crippling the company by wiping workstations and servers.[134][135] Vehicles are increasingly computerized, with engine timing,cruise control,anti-lock brakes, seat belt tensioners, door locks,airbagsandadvanced driver-assistance systemson many models. Additionally,connected carsmay use WiFi and Bluetooth to communicate with onboard consumer devices and the cell phone network.[136]Self-driving carsare expected to be even more complex. All of these systems carry some security risks, and such issues have gained wide attention.[137][138][139] Simple examples of risk include a maliciouscompact discbeing used as an attack vector,[140]and the car's onboard microphones being used for eavesdropping. However, if access is gained to a car's internalcontroller area network, the danger is much greater[136]– and in a widely publicized 2015 test, hackers remotely carjacked a vehicle from 10 miles away and drove it into a ditch.[141][142] Manufacturers are reacting in numerous ways, withTeslain 2016 pushing out some security fixesover the airinto its cars' computer systems.[143]In the area of autonomous vehicles, in September 2016 theUnited States Department of Transportationannounced some initial safety standards, and called for states to come up with uniform policies.[144][145][146] Additionally, e-Drivers' licenses are being developed using the same technology. For example, Mexico's licensing authority (ICV) has used a smart card platform to issue the first e-Drivers' licenses to the city ofMonterrey, in the state ofNuevo León.[147] Shipping companies[148]have adoptedRFID(Radio Frequency Identification) technology as an efficient, digitally secure,tracking device. Unlike abarcode, RFID can be read up to 20 feet away. RFID is used byFedEx[149]andUPS.[150] Government andmilitarycomputer systems are commonly attacked by activists[151][152][153]and foreign powers.[154][155][156][157]Local and regional government infrastructure such astraffic lightcontrols, police and intelligence agency communications,personnel records, as well as student records.[158] TheFBI,CIA, andPentagon, all utilize secure controlled access technology for any of their buildings. However, the use of this form of technology is spreading into the entrepreneurial world. More and more companies are taking advantage of the development of digitally secure controlled access technology. GE's ACUVision, for example, offers a single panel platform for access control, alarm monitoring and digital recording.[159] TheInternet of things(IoT) is the network of physical objects such as devices, vehicles, and buildings that areembeddedwithelectronics,software,sensors, andnetwork connectivitythat enables them to collect and exchange data.[160]Concerns have been raised that this is being developed without appropriate consideration of the security challenges involved.[161][162] While the IoT creates opportunities for more direct integration of the physical world into computer-based systems,[163][164]it also provides opportunities for misuse. In particular, as the Internet of Things spreads widely, cyberattacks are likely to become an increasingly physical (rather than simply virtual) threat.[165]If a front door's lock is connected to the Internet, and can be locked/unlocked from a phone, then a criminal could enter the home at the press of a button from a stolen or hacked phone. People could stand to lose much more than their credit card numbers in a world controlled by IoT-enabled devices. Thieves have also used electronic means to circumvent non-Internet-connected hotel door locks.[166] An attack aimed at physical infrastructure or human lives is often called a cyber-kinetic attack. As IoT devices and appliances become more widespread, the prevalence and potential damage of cyber-kinetic attacks can increase substantially. Medical deviceshave either been successfully attacked or had potentially deadly vulnerabilities demonstrated, including both in-hospital diagnostic equipment[167]and implanted devices includingpacemakers[168]andinsulin pumps.[169]There are many reports of hospitals and hospital organizations getting hacked, includingransomwareattacks,[170][171][172][173]Windows XPexploits,[174][175]viruses,[176][177]and data breaches of sensitive data stored on hospital servers.[178][171][179][180]On 28 December 2016 the USFood and Drug Administrationreleased its recommendations for how medicaldevice manufacturersshould maintain the security of Internet-connected devices – but no structure for enforcement.[181][182] In distributed generation systems, the risk of a cyber attack is real, according toDaily Energy Insider. An attack could cause a loss of power in a large area for a long period of time, and such an attack could have just as severe consequences as a natural disaster. The District of Columbia is considering creating a Distributed Energy Resources (DER) Authority within the city, with the goal being for customers to have more insight into their own energy use and giving the local electric utility,Pepco, the chance to better estimate energy demand. The D.C. proposal, however, would "allow third-party vendors to create numerous points of energy distribution, which could potentially create more opportunities for cyber attackers to threaten the electric grid."[183] Perhaps the most widely known digitally secure telecommunication device is theSIM(Subscriber Identity Module) card, a device that is embedded in most of the world's cellular devices before any service can be obtained. The SIM card is just the beginning of this digitally secure environment. The Smart Card Web Servers draft standard (SCWS) defines the interfaces to anHTTP serverin asmart card.[184]Tests are being conducted to secure OTA ("over-the-air") payment and credit card information from and to a mobile phone. Combination SIM/DVD devices are being developed through Smart Video Card technology which embeds aDVD-compliantoptical discinto the card body of a regular SIM card. Other telecommunication developments involving digital security includemobile signatures, which use the embedded SIM card to generate a legally bindingelectronic signature. Serious financial damage has been caused bysecurity breaches, but because there is no standard model for estimating the cost of an incident, the only data available is that which is made public by the organizations involved. "Several computer security consulting firms produce estimates of total worldwide losses attributable tovirusand worm attacks and to hostile digital acts in general. The 2003 loss estimates by these firms range from $13 billion (worms and viruses only) to $226 billion (for all forms of covert attacks). The reliability of these estimates is often challenged; the underlying methodology is basically anecdotal."[185] However, reasonable estimates of the financial cost of security breaches can actually help organizations make rational investment decisions. According to the classicGordon-Loeb Modelanalyzing the optimal investment level in information security, one can conclude that the amount a firm spends to protect information should generally be only a small fraction of the expected loss (i.e., theexpected valueof the loss resulting from a cyber/informationsecurity breach).[186] As withphysical security, the motivations for breaches of computer security vary between attackers. Some are thrill-seekers orvandals, some are activists, others are criminals looking for financial gain. State-sponsored attackers are now common and well resourced but started with amateurs such as Markus Hess who hacked for theKGB, as recounted byClifford StollinThe Cuckoo's Egg. Attackers motivations can vary for all types of attacks from pleasure to political goals.[15]For example, hacktivists may target a company or organization that carries out activities they do not agree with. This would be to create bad publicity for the company by having its website crash. High capability hackers, often with larger backing or state sponsorship, may attack based on the demands of their financial backers. These attacks are more likely to attempt more serious attack. An example of a more serious attack was the2015 Ukraine power grid hack, which reportedly utilised the spear-phising, destruction of files, and denial-of-service attacks to carry out the full attack.[187][188] Additionally, recent attacker motivations can be traced back to extremist organizations seeking to gain political advantage or disrupt social agendas.[189]The growth of the internet, mobile technologies, and inexpensive computing devices have led to a rise in capabilities but also to the risk to environments that are deemed as vital to operations. All critical targeted environments are susceptible to compromise and this has led to a series of proactive studies on how to migrate the risk by taking into consideration motivations by these types of actors. Several stark differences exist between the hacker motivation and that ofnation stateactors seeking to attack based on an ideological preference.[190] A key aspect of threat modeling for any system is identifying the motivations behind potential attacks and the individuals or groups likely to carry them out. The level and detail of security measures will differ based on the specific system being protected. For instance, a home personal computer, a bank, and a classified military network each face distinct threats, despite using similar underlying technologies.[191] Computer security incident managementis an organized approach to addressing and managing the aftermath of a computer security incident or compromise with the goal of preventing a breach or thwarting a cyberattack. An incident that is not identified and managed at the time of intrusion typically escalates to a more damaging event such as adata breachor system failure. The intended outcome of a computer security incident response plan is to contain the incident, limit damage and assist recovery to business as usual. Responding to compromises quickly can mitigate exploited vulnerabilities, restore services and processes and minimize losses.[192]Incident response planning allows an organization to establish a series of best practices to stop an intrusion before it causes damage. Typical incident response plans contain a set of written instructions that outline the organization's response to a cyberattack. Without a documented plan in place, an organization may not successfully detect an intrusion or compromise and stakeholders may not understand their roles, processes and procedures during an escalation, slowing the organization's response and resolution. There are four key components of a computer security incident response plan: Some illustrative examples of different types of computer security breaches are given below. In 1988, 60,000 computers were connected to the Internet, and most were mainframes, minicomputers and professional workstations. On 2 November 1988, many started to slow down, because they were running a malicious code that demanded processor time and that spread itself to other computers – the first internetcomputer worm.[194]The software was traced back to 23-year-oldCornell Universitygraduate studentRobert Tappan Morriswho said "he wanted to count how many machines were connected to the Internet".[194] In 1994, over a hundred intrusions were made by unidentified crackers into theRome Laboratory, the US Air Force's main command and research facility. Usingtrojan horses, hackers were able to obtain unrestricted access to Rome's networking systems and remove traces of their activities. The intruders were able to obtain classified files, such as air tasking order systems data and furthermore able to penetrate connected networks ofNational Aeronautics and Space Administration's Goddard Space Flight Center, Wright-Patterson Air Force Base, some Defense contractors, and other private sector organizations, by posing as a trusted Rome center user.[195] In early 2007, American apparel and home goods companyTJXannounced that it was the victim of anunauthorized computer systems intrusion[196]and that the hackers had accessed a system that stored data oncredit card,debit card,check, and merchandise return transactions.[197] In 2010, the computer worm known asStuxnetreportedly ruined almost one-fifth of Iran'snuclear centrifuges.[198]It did so by disrupting industrialprogrammable logic controllers(PLCs) in a targeted attack. This is generally believed to have been launched by Israel and the United States to disrupt Iran's nuclear program[199][200][201][202]– although neither has publicly admitted this. In early 2013, documents provided byEdward Snowdenwere published byThe Washington PostandThe Guardian[203][204]exposing the massive scale ofNSAglobal surveillance. There were also indications that the NSA may have inserted a backdoor in aNISTstandard for encryption.[205]This standard was later withdrawn due to widespread criticism.[206]The NSA additionally were revealed to have tapped the links betweenGoogle's data centers.[207] A Ukrainian hacker known asRescatorbroke intoTarget Corporationcomputers in 2013, stealing roughly 40 million credit cards,[208]and thenHome Depotcomputers in 2014, stealing between 53 and 56 million credit card numbers.[209]Warnings were delivered at both corporations, but ignored; physical security breaches usingself checkout machinesare believed to have played a large role. "The malware utilized is absolutely unsophisticated and uninteresting," says Jim Walter, director of threat intelligence operations at security technology company McAfee – meaning that the heists could have easily been stopped by existingantivirus softwarehad administrators responded to the warnings. The size of the thefts has resulted in major attention from state and Federal United States authorities and the investigation is ongoing. In April 2015, theOffice of Personnel Managementdiscovered it had been hackedmore than a year earlier in a data breach, resulting in the theft of approximately 21.5 million personnel records handled by the office.[210]The Office of Personnel Management hack has been described by federal officials as among the largest breaches of government data in the history of the United States.[211]Data targeted in the breach includedpersonally identifiable informationsuch asSocial Security numbers, names, dates and places of birth, addresses, and fingerprints of current and former government employees as well as anyone who had undergone a government background check.[212][213]It is believed the hack was perpetrated by Chinese hackers.[214] In July 2015, a hacker group is known as The Impact Team successfully breached the extramarital relationship website Ashley Madison, created by Avid Life Media. The group claimed that they had taken not only company data but user data as well. After the breach, The Impact Team dumped emails from the company's CEO, to prove their point, and threatened to dump customer data unless the website was taken down permanently.[215]When Avid Life Media did not take the site offline the group released two more compressed files, one 9.7GB and the second 20GB. After the second data dump, Avid Life Media CEO Noel Biderman resigned; but the website remained to function. In June 2021, the cyber attack took down the largest fuel pipeline in the U.S. and led to shortages across the East Coast.[216] International legal issues of cyber attacks are complicated in nature. There is no global base of common rules to judge, and eventually punish, cybercrimes and cybercriminals - and where security firms or agencies do locate the cybercriminal behind the creation of a particular piece ofmalwareor form ofcyber attack, often the local authorities cannot take action due to lack of laws under which to prosecute.[217][218]Provingattribution for cybercrimes and cyberattacksis also a major problem for all law enforcement agencies. "Computer virusesswitch from one country to another, from one jurisdiction to another – moving around the world, using the fact that we don't have the capability to globally police operations like this. So the Internet is as if someone [had] given free plane tickets to all the online criminals of the world."[217]The use of techniques such asdynamic DNS,fast fluxandbullet proof serversadd to the difficulty of investigation and enforcement. The role of the government is to makeregulationsto force companies and organizations to protect their systems, infrastructure and information from any cyberattacks, but also to protect its own national infrastructure such as the nationalpower-grid.[219] The government's regulatory role incyberspaceis complicated. For some, cyberspace was seen as avirtual spacethat was to remain free of government intervention, as can be seen in many of today's libertarianblockchainandbitcoindiscussions.[220] Many government officials and experts think that the government should do more and that there is a crucial need for improved regulation, mainly due to the failure of the private sector to solve efficiently the cybersecurity problem.R. Clarkesaid during a panel discussion at theRSA Security ConferenceinSan Francisco, he believes that the "industry only responds when you threaten regulation. If the industry doesn't respond (to the threat), you have to follow through."[221]On the other hand, executives from the private sector agree that improvements are necessary, but think that government intervention would affect their ability to innovate efficiently. Daniel R. McCarthy analyzed this public-private partnership in cybersecurity and reflected on the role of cybersecurity in the broader constitution of political order.[222] On 22 May 2020, the UN Security Council held its second ever informal meeting on cybersecurity to focus on cyber challenges tointernational peace. According to UN Secretary-GeneralAntónio Guterres, new technologies are too often used to violate rights.[223] Many different teams and organizations exist, including: On 14 April 2016, theEuropean Parliamentand theCouncil of the European Unionadopted theGeneral Data Protection Regulation(GDPR). The GDPR, which came into force on 25 May 2018, grants individuals within the European Union (EU) and the European Economic Area (EEA) the right to theprotection of personal data. The regulation requires that any entity that processes personal data incorporate data protection by design and by default. It also requires that certain organizations appoint a Data Protection Officer (DPO). The IT Security AssociationTeleTrusTexist inGermanysince June 1986, which is an international competence network for IT security. Most countries have their own computer emergency response team to protect network security. Since 2010, Canada has had a cybersecurity strategy.[229][230]This functions as a counterpart document to the National Strategy and Action Plan for Critical Infrastructure.[231]The strategy has three main pillars: securing government systems, securing vital private cyber systems, and helping Canadians to be secure online.[230][231]There is also a Cyber Incident Management Framework to provide a coordinated response in the event of a cyber incident.[232][233] TheCanadian Cyber Incident Response Centre(CCIRC) is responsible for mitigating and responding to threats to Canada's critical infrastructure and cyber systems. It provides support to mitigate cyber threats, technical support to respond & recover from targeted cyber attacks, and provides online tools for members of Canada's critical infrastructure sectors.[234]It posts regular cybersecurity bulletins[235]& operates an online reporting tool where individuals and organizations can report a cyber incident.[236] To inform the general public on how to protect themselves online, Public Safety Canada has partnered with STOP.THINK.CONNECT, a coalition of non-profit, private sector, and government organizations,[237]and launched the Cyber Security Cooperation Program.[238][239]They also run the GetCyberSafe portal for Canadian citizens, and Cyber Security Awareness Month during October.[240] Public Safety Canada aims to begin an evaluation of Canada's cybersecurity strategy in early 2015.[231] Australian federal governmentannounced an $18.2 million investment to fortify thecybersecurityresilience of small and medium enterprises (SMEs) and enhance their capabilities in responding to cyber threats. This financial backing is an integral component of the soon-to-be-unveiled2023-2030 Australian Cyber Security Strategy, slated for release within the current week. A substantial allocation of $7.2 million is earmarked for the establishment of a voluntary cyber health check program, facilitating businesses in conducting a comprehensive and tailored self-assessment of their cybersecurity upskill. This avant-garde health assessment serves as a diagnostic tool, enabling enterprises to ascertain the robustness ofAustralia's cyber security regulations. Furthermore, it affords them access to a repository of educational resources and materials, fostering the acquisition of skills necessary for an elevated cybersecurity posture. This groundbreaking initiative was jointly disclosed by Minister for Cyber SecurityClare O'Neiland Minister for Small BusinessJulie Collins.[241] Some provisions for cybersecurity have been incorporated into rules framed under the Information Technology Act 2000.[242] TheNational Cyber Security Policy 2013is a policy framework by the Ministry of Electronics and Information Technology (MeitY) which aims to protect the public and private infrastructure from cyberattacks, and safeguard "information, such as personal information (of web users), financial and banking information and sovereign data".CERT- Inis the nodal agency which monitors the cyber threats in the country. The post ofNational Cyber Security Coordinatorhas also been created in thePrime Minister's Office (PMO). The Indian Companies Act 2013 has also introduced cyber law and cybersecurity obligations on the part of Indian directors. Some provisions for cybersecurity have been incorporated into rules framed under the Information Technology Act 2000 Update in 2013.[243] Following cyberattacks in the first half of 2013, when the government, news media, television stations, and bank websites were compromised, the national government committed to the training of 5,000 new cybersecurity experts by 2017. The South Korean government blamed its northern counterpart for these attacks, as well as incidents that occurred in 2009, 2011,[244]and 2012, but Pyongyang denies the accusations.[245] TheUnited Stateshas its first fully formed cyber plan in 15 years, as a result of the release of this National Cyber plan.[246]In this policy, the US says it will: Protect the country by keeping networks, systems, functions, and data safe; Promote American wealth by building a strong digital economy and encouraging strong domestic innovation; Peace and safety should be kept by making it easier for the US to stop people from using computer tools for bad things, working with friends and partners to do this; and increase the United States' impact around the world to support the main ideas behind an open, safe, reliable, and compatible Internet.[247] The new U.S. cyber strategy[248]seeks to allay some of those concerns by promoting responsible behavior incyberspace, urging nations to adhere to a set of norms, both through international law and voluntary standards. It also calls for specific measures to harden U.S. government networks from attacks, like the June 2015 intrusion into theU.S. Office of Personnel Management(OPM), which compromised the records of about 4.2 million current and former government employees. And the strategy calls for the U.S. to continue to name and shame bad cyber actors, calling them out publicly for attacks when possible, along with the use of economic sanctions and diplomatic pressure.[249] The 198618 U.S.C.§ 1030, theComputer Fraud and Abuse Actis the key legislation. It prohibits unauthorized access or damage ofprotected computersas defined in18 U.S.C.§ 1030(e)(2). Although various other measures have been proposed[250][251]– none have succeeded. In 2013,executive order13636Improving Critical Infrastructure Cybersecuritywas signed, which prompted the creation of theNIST Cybersecurity Framework. In response to theColonial Pipeline ransomware attack[252]PresidentJoe Bidensigned Executive Order 14028[253]on May 12, 2021, to increase software security standards for sales to the government, tighten detection and security on existing systems, improve information sharing and training, establish a Cyber Safety Review Board, and improve incident response. TheGeneral Services Administration(GSA) has[when?]standardized thepenetration testservice as a pre-vetted support service, to rapidly address potential vulnerabilities, and stop adversaries before they impact US federal, state and local governments. These services are commonly referred to as Highly Adaptive Cybersecurity Services (HACS). TheDepartment of Homeland Securityhas a dedicated division responsible for the response system,risk managementprogram and requirements for cybersecurity in the United States called theNational Cyber Security Division.[254][255]The division is home to US-CERT operations and the National Cyber Alert System.[255]The National Cybersecurity and Communications Integration Center brings together government organizations responsible for protecting computer networks and networked infrastructure.[256] The third priority of the FBI is to: "Protect the United States against cyber-based attacks and high-technology crimes",[257]and they, along with theNational White Collar Crime Center(NW3C), and theBureau of Justice Assistance(BJA) are part of the multi-agency task force, TheInternet Crime Complaint Center, also known as IC3.[258] In addition to its own specific duties, the FBI participates alongside non-profit organizations such asInfraGard.[259][260] TheComputer Crime and Intellectual Property Section(CCIPS) operates in theUnited States Department of Justice Criminal Division. The CCIPS is in charge of investigatingcomputer crimeandintellectual propertycrime and is specialized in the search and seizure ofdigital evidencein computers andnetworks.[261]In 2017, CCIPS published A Framework for a Vulnerability Disclosure Program for Online Systems to help organizations "clearly describe authorized vulnerability disclosure and discovery conduct, thereby substantially reducing the likelihood that such described activities will result in a civil or criminal violation of law under the Computer Fraud and Abuse Act (18 U.S.C. § 1030)."[262] TheUnited States Cyber Command, also known as USCYBERCOM, "has the mission to direct, synchronize, and coordinate cyberspace planning and operations to defend and advance national interests in collaboration with domestic and international partners."[263]It has no role in the protection of civilian networks.[264][265] The U.S.Federal Communications Commission's role in cybersecurity is to strengthen the protection of critical communications infrastructure, to assist in maintaining the reliability of networks during disasters, to aid in swift recovery after, and to ensure that first responders have access to effective communications services.[266] TheFood and Drug Administrationhas issued guidance for medical devices,[267]and theNational Highway Traffic Safety Administration[268]is concerned with automotive cybersecurity. After being criticized by theGovernment Accountability Office,[269]and following successful attacks on airports and claimed attacks on airplanes, theFederal Aviation Administrationhas devoted funding to securing systems on board the planes of private manufacturers, and theAircraft Communications Addressing and Reporting System.[270]Concerns have also been raised about the futureNext Generation Air Transportation System.[271] The US Department of Defense (DoD) issued DoD Directive 8570 in 2004, supplemented by DoD Directive 8140, requiring all DoD employees and all DoD contract personnel involved in information assurance roles and activities to earn and maintain various industry Information Technology (IT) certifications in an effort to ensure that all DoD personnel involved in network infrastructure defense have minimum levels of IT industry recognized knowledge, skills and abilities (KSA). Andersson and Reimers (2019) report these certifications range from CompTIA's A+ and Security+ through the ICS2.org's CISSP, etc.[272] Computer emergency response teamis a name given to expert groups that handle computer security incidents. In the US, two distinct organizations exist, although they do work closely together. In the context ofU.S. nuclear power plants, theU.S. Nuclear Regulatory Commission (NRC)outlines cybersecurity requirements under10 CFR Part 73, specifically in §73.54.[274] TheNuclear Energy Institute's NEI 08-09 document,Cyber Security Plan for Nuclear Power Reactors,[275]outlines a comprehensive framework forcybersecurityin thenuclear power industry. Drafted with input from theU.S. NRC, this guideline is instrumental in aidinglicenseesto comply with theCode of Federal Regulations (CFR), which mandates robust protection of digital computers and equipment and communications systems at nuclear power plants against cyber threats.[276] There is growing concern that cyberspace will become the next theater of warfare. As Mark Clayton fromThe Christian Science Monitorwrote in a 2015 article titled "The New Cyber Arms Race": In the future, wars will not just be fought by soldiers with guns or with planes that drop bombs. They will also be fought with the click of a mouse a half a world away that unleashes carefully weaponized computer programs that disrupt or destroy critical industries like utilities, transportation, communications, and energy. Such attacks could also disable military networks that control the movement of troops, the path of jet fighters, the command and control of warships.[277] This has led to new terms such ascyberwarfareandcyberterrorism. TheUnited States Cyber Commandwas created in 2009[278]and many other countrieshave similar forces. There are a few critical voices that question whether cybersecurity is as significant a threat as it is made out to be.[279][280][281] Cybersecurity is a fast-growing field ofITconcerned with reducing organizations' risk of hack or data breaches.[282]According to research from the Enterprise Strategy Group, 46% of organizations say that they have a "problematic shortage" of cybersecurity skills in 2016, up from 28% in 2015.[283]Commercial, government and non-governmental organizations all employ cybersecurity professionals. The fastest increases in demand for cybersecurity workers are in industries managing increasing volumes of consumer data such as finance, health care, and retail.[284]However, the use of the termcybersecurityis more prevalent in government job descriptions.[285] Typical cybersecurity job titles and descriptions include:[286] Student programs are also available for people interested in beginning a career in cybersecurity.[290][291]Meanwhile, a flexible and effective option for information security professionals of all experience levels to keep studying is online security training, including webcasts.[292][293]A wide range of certified courses are also available.[294] In the United Kingdom, a nationwide set of cybersecurity forums, known as theU.K Cyber Security Forum, were established supported by the Government's cybersecurity strategy[295]in order to encourage start-ups and innovation and to address the skills gap[296]identified by theU.K Government. In Singapore, theCyber Security Agencyhas issued a Singapore Operational Technology (OT) Cybersecurity Competency Framework (OTCCF). The framework defines emerging cybersecurity roles in Operational Technology. The OTCCF was endorsed by theInfocomm Media Development Authority(IMDA). It outlines the different OT cybersecurity job positions as well as the technical skills and core competencies necessary. It also depicts the many career paths available, including vertical and lateral advancement opportunities.[297] The following terms used with regards to computer security are explained below: Since theInternet's arrival and with the digital transformation initiated in recent years, the notion of cybersecurity has become a familiar subject in both our professional and personal lives. Cybersecurity and cyber threats have been consistently present for the last 60 years of technological change. In the 1970s and 1980s, computer security was mainly limited toacademiauntil the conception of the Internet, where, with increased connectivity, computer viruses and network intrusions began to take off. After the spread of viruses in the 1990s, the 2000s marked the institutionalization of organized attacks such asdistributed denial of service.[301]This led to the formalization of cybersecurity as a professional discipline.[302] TheApril 1967 sessionorganized byWillis Wareat theSpring Joint Computer Conference, and the later publication of theWare Report, were foundational moments in the history of the field of computer security.[303]Ware's work straddled the intersection of material, cultural, political, and social concerns.[303] A 1977NISTpublication[304]introduced theCIA triadof confidentiality, integrity, and availability as a clear and simple way to describe key security goals.[305]While still relevant, many more elaborate frameworks have since been proposed.[306][307] However, in the 1970s and 1980s, there were no grave computer threats because computers and the internet were still developing, and security threats were easily identifiable. More often, threats came from malicious insiders who gained unauthorized access to sensitive documents and files. Although malware and network breaches existed during the early years, they did not use them for financial gain. By the second half of the 1970s, established computer firms likeIBMstarted offering commercial access control systems and computer security software products.[308] One of the earliest examples of an attack on a computer network was thecomputer wormCreeperwritten by Bob Thomas atBBN, which propagated through theARPANETin 1971.[309]The program was purely experimental in nature and carried no malicious payload. A later program,Reaper, was created byRay Tomlinsonin 1972 and used to destroy Creeper.[citation needed] Between September 1986 and June 1987, a group of German hackers performed the first documented case of cyber espionage.[310]The group hacked into American defense contractors, universities, and military base networks and sold gathered information to the Soviet KGB. The group was led byMarkus Hess, who was arrested on 29 June 1987. He was convicted of espionage (along with two co-conspirators) on 15 Feb 1990. In 1988, one of the first computer worms, called theMorris worm, was distributed via the Internet. It gained significant mainstream media attention.[311] Netscapestarted developing the protocolSSL, shortly after the National Center for Supercomputing Applications (NCSA) launched Mosaic 1.0, the first web browser, in 1993.[312][313]Netscape had SSL version 1.0 ready in 1994, but it was never released to the public due to many serious security vulnerabilities.[312]However, in 1995, Netscape launched Version 2.0.[314] TheNational Security Agency(NSA) is responsible for theprotectionof U.S. information systems and also for collecting foreign intelligence.[315]The agency analyzes commonly used software and system configurations to find security flaws, which it can use for offensive purposes against competitors of the United States.[316] NSA contractors created and soldclick-and-shootattack tools to US agencies and close allies, but eventually, the tools made their way to foreign adversaries.[317]In 2016, NSAs own hacking tools were hacked, and they have been used by Russia and North Korea.[citation needed]NSA's employees and contractors have been recruited at high salaries by adversaries, anxious to compete incyberwarfare.[citation needed]In 2007, the United States andIsraelbegan exploiting security flaws in theMicrosoft Windowsoperating system to attack and damage equipment used in Iran to refine nuclear materials. Iran responded by heavily investing in their own cyberwarfare capability, which it began using against the United States.[316]	https://en.wikipedia.org/wiki/Computer_insecurity
Acomputer virus[1]is a type ofmalwarethat, when executed, replicates itself by modifying othercomputer programsandinsertingits owncodeinto those programs.[2][3]If this replication succeeds, the affected areas are then said to be "infected" with a computer virus, a metaphor derived from biologicalviruses.[4] Computer viruses generally require ahost program.[5]The virus writes its own code into the host program. When the program runs, the written virus program is executed first, causing infection and damage. By contrast, acomputer wormdoes not need a host program, as it is an independent program or code chunk. Therefore, it is not restricted by thehost program, but can run independently and actively carry out attacks.[6][7] Virus writers usesocial engineeringdeceptionsand exploit detailed knowledge ofsecurity vulnerabilitiesto initially infect systems and to spread the virus. Viruses use complex anti-detection/stealth strategies to evadeantivirus software.[8]Motives for creating viruses can include seekingprofit(e.g., withransomware), desire to send a political message, personal amusement, to demonstrate that a vulnerability exists in software, forsabotageanddenial of service, or simply because they wish to explorecybersecurityissues,artificial lifeandevolutionary algorithms.[9] As of 2013, computer viruses caused billions of dollars' worth of economic damage each year.[10]In response, an industry ofantivirus softwarehas cropped up, selling or freely distributing virus protection to users of variousoperating systems.[11] The first academic work on the theory of self-replicating computer programs was done in 1949 byJohn von Neumannwho gave lectures at theUniversity of Illinoisabout the "Theory and Organization of ComplicatedAutomata". The work of von Neumann was later published as the "Theory of self-reproducing automata". In his essay von Neumann described how a computer program could be designed to reproduce itself.[12]Von Neumann's design for a self-reproducing computer program is considered the world's first computer virus, and he is considered to be the theoretical "father" of computer virology.[13] In 1972, Veith Risak directly building on von Neumann's work onself-replication, published his article "Selbstreproduzierende Automaten mit minimaler Informationsübertragung" (Self-reproducing automata with minimal information exchange).[14]The article describes a fully functional virus written inassemblerprogramming language for a SIEMENS 4004/35 computer system. In 1980, Jürgen Kraus wrote hisDiplomthesis "Selbstreproduktion bei Programmen" (Self-reproduction of programs) at theUniversity of Dortmund.[15]In his work Kraus postulated that computer programs can behave in a way similar to biological viruses. TheCreeper viruswas first detected onARPANET, the forerunner of theInternet, in the early 1970s.[16]Creeper was an experimental self-replicating program written by Bob Thomas atBBN Technologiesin 1971.[17]Creeper used the ARPANET to infectDECPDP-10computers running theTENEXoperating system.[18]Creeper gained access via the ARPANET and copied itself to the remote system where the message, "I'M THE CREEPER. CATCH ME IF YOU CAN!" was displayed.[19]TheReaperprogram was created to delete Creeper.[20] In 1982, a program called "Elk Cloner" was the first personal computer virus to appear "in the wild"—that is, outside the single computer or computer lab where it was created.[21]Written in 1981 byRichard Skrenta, a ninth grader atMount Lebanon High SchoolnearPittsburgh, it attached itself to theApple DOS3.3 operating system and spread viafloppy disk.[21]On its 50th use theElk Clonervirus would be activated, infecting the personal computer and displaying a short poem beginning "Elk Cloner: The program with a personality." In 1984,Fred Cohenfrom theUniversity of Southern Californiawrote his paper "Computer Viruses – Theory and Experiments".[22]It was the first paper to explicitly call a self-reproducing program a "virus", a term introduced by Cohen's mentorLeonard Adleman.[23]In 1987, Cohen published a demonstration that there is noalgorithmthat can perfectly detect all possible viruses.[24]Cohen's theoreticalcompression virus[25]was an example of a virus which was not malicious software (malware), but was putatively benevolent (well-intentioned). However,antivirusprofessionals do not accept the concept of "benevolent viruses", as any desired function can be implemented without involving a virus (automatic compression, for instance, is available underWindowsat the choice of the user). Any virus will by definition make unauthorised changes to a computer, which is undesirable even if no damage is done or intended. The first page ofDr Solomon's Virus Encyclopaediaexplains the undesirability of viruses, even those that do nothing but reproduce.[26][27] An article that describes "useful virus functionalities" was published byJ. B. Gunnunder the title "Use of virus functions to provide a virtualAPLinterpreter under user control" in 1984.[28]The firstIBM PC compatiblevirus in the "wild" was aboot sectorvirus dubbed(c)Brain,[29]created in 1986 and was released in 1987 by Amjad Farooq Alvi and Basit Farooq Alvi inLahore, Pakistan, reportedly to deter unauthorized copying of the software they had written.[30] The first virus to specifically targetMicrosoft Windows,WinVirwas discovered in April 1992, two years after the release ofWindows 3.0.[31]The virus did not contain anyWindows APIcalls, instead relying onDOS interrupts. A few years later, in February 1996, Australian hackers from the virus-writing crew VLAD created theBizatchvirus (also known as "Boza" virus), which was the first known virus to specifically targetWindows 95.[32]This virus attacked the new portable executable (PE) files introduced in Windows 95.[33]In late 1997 the encrypted, memory-resident stealth virusWin32.Cabanaswas released—the first known virus that targetedWindows NT(it was also able to infect Windows 3.0 and Windows 9x hosts).[34] Evenhome computerswere affected by viruses. The first one to appear on theAmigawas a boot sector virus calledSCA virus, which was detected in November 1987.[35]By 1988, onesysopreportedly found that viruses infected 15% of the software available for download on his BBS.[36] A computer virus generally contains three parts: the infection mechanism, which finds and infects new files, the payload, which is the malicious code to execute, and the trigger, which determines when to activate the payload.[37] Virus phases is thelife cycleof the computer virus, described by using an analogy tobiology. This life cycle can be divided into four phases: Computer viruses infect a variety of different subsystems on their host computers and software.[45]One manner of classifying viruses is to analyze whether they reside inbinary executables(such as.EXEor.COM files), data files (such asMicrosoft Worddocuments orPDF files), or in theboot sectorof the host'shard drive(or some combination of all of these).[46][47] Amemory-resident virus(or simply "resident virus") installs itself as part of the operating system when executed, after which it remains inRAMfrom the time the computer is booted up to when it is shut down. Resident viruses overwriteinterrupt handlingcode or otherfunctions, and when the operating system attempts to access the target file or disk sector, the virus code intercepts the request and redirects thecontrol flowto the replication module, infecting the target. In contrast, anon-memory-resident virus(or "non-resident virus"), when executed, scans the disk for targets, infects them, and then exits (i.e. it does not remain in memory after it is done executing).[48] Many common applications, such asMicrosoft OutlookandMicrosoft Word, allowmacroprograms to be embedded in documents or emails, so that the programs may be run automatically when the document is opened. Amacro virus(or "document virus") is a virus that is written in amacro languageand embedded into these documents so that when users open the file, the virus code is executed, and can infect the user's computer. This is one of the reasons that it is dangerous to open unexpected or suspiciousattachmentsine-mails.[49][50]While not opening attachments in e-mails from unknown persons or organizations can help to reduce the likelihood of contracting a virus, in some cases, the virus is designed so that the e-mail appears to be from a reputable organization (e.g., a major bank or credit card company). Boot sector virusesspecifically target theboot sectorand/or theMaster Boot Record[51](MBR) of the host'shard disk drive,solid-state drive, or removable storage media (flash drives,floppy disks, etc.).[52] The most common way of transmission of computer viruses in boot sector is physical media. When reading theVBRof the drive, the infected floppy disk or USBflash driveconnected to the computer will transfer data, and then modify or replace the existing boot code. The next time a user tries to start the desktop, the virus will immediately load and run as part of the master boot record.[53] Email viruses are viruses that intentionally, rather than accidentally, use the email system to spread. While virus infected files may be accidentally sent asemail attachments, email viruses are aware of email system functions. They generally target a specific type of email system (Microsoft Outlookis the most commonly used), harvest email addresses from various sources, and may append copies of themselves to all email sent, or may generate email messages containing copies of themselves as attachments.[54] To avoid detection by users, some viruses employ different kinds ofdeception. Some old viruses, especially on theDOSplatform, make sure that the "last modified" date of a host file stays the same when the file is infected by the virus. This approach does not fool antivirussoftware, however, especially those which maintain and datecyclic redundancy checkson file changes.[55]Some viruses can infect files without increasing their sizes or damaging the files. They accomplish this by overwriting unused areas of executable files. These are calledcavity viruses. For example, theCIH virus, or Chernobyl Virus, infectsPortable Executablefiles. Because those files have many empty gaps, the virus, which was 1KBin length, did not add to the size of the file.[56]Some viruses try to avoid detection by killing the tasks associated with antivirus software before it can detect them (for example,Conficker). A Virus may also hide its presence using arootkitby not showing itself on the list of systemprocessesor by disguising itself within a trusted process.[57]In the 2010s, as computers and operating systems grow larger and more complex, old hiding techniques need to be updated or replaced. Defending a computer against viruses may demand that a file system migrate towards detailed and explicit permission for every kind of file access.[citation needed]In addition, only a small fraction of known viruses actually cause real incidents, primarily because many viruses remain below the theoretical epidemic threshold.[58] While some kinds of antivirus software employ various techniques to counter stealth mechanisms, once the infection occurs any recourse to "clean" the system is unreliable. In Microsoft Windows operating systems, theNTFS file systemis proprietary. This leaves antivirus software little alternative but to send a "read" request to Windows files that handle such requests. Some viruses trick antivirus software by intercepting its requests to the operating system. A virus can hide by intercepting the request to read the infected file, handling the request itself, and returning an uninfected version of the file to the antivirus software. The interception can occur bycode injectionof the actual operating system files that would handle the read request. Thus, an antivirus software attempting to detect the virus will either not be permitted to read the infected file, or, the "read" request will be served with the uninfected version of the same file.[59] The only reliable method to avoid "stealth" viruses is to boot from a medium that is known to be "clear". Security software can then be used to check the dormant operating system files. Most security software relies on virus signatures, or they employheuristics.[60][61]Security software may also use a database of file "hashes" for Windows OS files, so the security software can identify altered files, and request Windows installation media to replace them with authentic versions. In older versions of Windows, filecryptographic hash functionsof Windows OS files stored in Windows—to allow file integrity/authenticity to be checked—could be overwritten so that theSystem File Checkerwould report that altered system files are authentic, so using file hashes to scan for altered files would not always guarantee finding an infection.[62] Most modern antivirus programs try to find virus-patterns inside ordinary programs by scanning them for so-calledvirus signatures.[63]Different antivirus programs will employ different search methods when identifying viruses. If a virus scanner finds such a pattern in a file, it will perform other checks to make sure that it has found the virus, and not merely a coincidental sequence in an innocent file, before it notifies the user that the file is infected. The user can then delete, or (in some cases) "clean" or "heal" the infected file. Some viruses employ techniques that make detection by means of signatures difficult but probably not impossible. These viruses modify their code on each infection. That is, each infected file contains a different variant of the virus.[citation needed] One method of evading signature detection is to use simpleencryptionto encipher (encode) the body of the virus, leaving only the encryption module and a staticcryptographic keyincleartextwhich does not change from one infection to the next.[64]In this case, the virus consists of a small decrypting module and an encrypted copy of the virus code. If the virus is encrypted with a different key for each infected file, the only part of the virus that remains constant is the decrypting module, which would (for example) be appended to the end. In this case, a virus scanner cannot directly detect the virus using signatures, but it can still detect the decrypting module, which still makes indirect detection of the virus possible. Since these would be symmetric keys, stored on the infected host, it is entirely possible to decrypt the final virus, but this is probably not required, sinceself-modifying codeis such a rarity that finding some may be reason enough for virus scanners to at least "flag" the file as suspicious.[citation needed]An old but compact way will be the use of arithmetic operation like addition or subtraction and the use of logical conditions such asXORing,[65]where each byte in a virus is with a constant so that the exclusive-or operation had only to be repeated for decryption. It is suspicious for a code to modify itself, so the code to do the encryption/decryption may be part of the signature in many virus definitions.[citation needed]A simpler older approach did not use a key, where the encryption consisted only of operations with no parameters, like incrementing and decrementing, bitwise rotation, arithmetic negation, and logical NOT.[65]Some viruses, called polymorphic viruses, will employ a means of encryption inside an executable in which the virus is encrypted under certain events, such as the virus scanner being disabled for updates or the computer beingrebooted.[66]This is calledcryptovirology. Polymorphic codewas the first technique that posed a seriousthreatto virus scanners. Just like regular encrypted viruses, a polymorphic virus infects files with an encrypted copy of itself, which is decoded by adecryptionmodule. In the case of polymorphic viruses, however, this decryption module is also modified on each infection. A well-written polymorphic virus therefore has no parts which remain identical between infections, making it very difficult to detect directly using "signatures".[67][68]Antivirus software can detect it by decrypting the viruses using anemulator, or bystatistical pattern analysisof the encrypted virus body. To enable polymorphic code, the virus has to have apolymorphic engine(also called "mutating engine" or "mutationengine") somewhere in its encrypted body. Seepolymorphic codefor technical detail on how such engines operate.[69] Some viruses employ polymorphic code in a way that constrains the mutation rate of the virus significantly. For example, a virus can be programmed to mutate only slightly over time, or it can be programmed to refrain from mutating when it infects a file on a computer that already contains copies of the virus. The advantage of using such slow polymorphic code is that it makes it more difficult for antivirus professionals and investigators to obtain representative samples of the virus, because "bait" files that are infected in one run will typically contain identical or similar samples of the virus. This will make it more likely that the detection by the virus scanner will be unreliable, and that some instances of the virus may be able to avoid detection. To avoid being detected by emulation, some viruses rewrite themselves completely each time they are to infect new executables. Viruses that utilize this technique are said to be inmetamorphic code. To enable metamorphism, a "metamorphic engine" is needed. A metamorphic virus is usually very large and complex. For example,W32/Simileconsisted of over 14,000 lines ofassembly languagecode, 90% of which is part of the metamorphic engine.[70][71] Damage is due to causing system failure, corrupting data, wasting computer resources, increasing maintenance costs or stealing personal information.[10]Even though no antivirus software can uncover all computer viruses (especially new ones), computer security researchers are actively searching for new ways to enable antivirus solutions to more effectively detect emerging viruses, before they become widely distributed.[72] Apower virusis a computer program that executes specific machine code to reach the maximumCPU power dissipation(thermal energyoutput for thecentral processing units).[73]Computer cooling apparatus are designed to dissipate power up to thethermal design power, rather than maximum power, and a power virus could cause the system to overheat if it does not have logic to stop the processor. This may cause permanent physical damage. Power viruses can be malicious, but are often suites of test software used forintegration testingand thermal testing of computer components during the design phase of a product, or for productbenchmarking.[74] Stability testapplications are similar programs which have the same effect as power viruses (high CPU usage) but stay under the user's control. They are used for testing CPUs, for example, whenoverclocking.Spinlockin a poorly written program may cause similar symptoms, if it lasts sufficiently long. Different micro-architectures typically require different machine code to hit their maximum power. Examples of such machine code do not appear to be distributed in CPU reference materials.[75] As software is often designed with security features to prevent unauthorized use of system resources, many viruses must exploit and manipulatesecurity bugs, which aresecurity defectsin a system or application software, to spread themselves and infect other computers.Software developmentstrategies that produce large numbers of "bugs" will generally also produce potentialexploitable"holes" or "entrances" for the virus. To replicate itself, a virus must be permitted to execute code and write to memory. For this reason, many viruses attach themselves toexecutable filesthat may be part of legitimate programs (seecode injection). If a user attempts to launch an infected program, the virus' code may be executed simultaneously.[76]In operating systems that usefile extensionsto determine program associations (such as Microsoft Windows), the extensions may be hidden from the user by default. This makes it possible to create a file that is of a different type than it appears to the user. For example, an executable may be created and named "picture.png.exe", in which the user sees only "picture.png" and therefore assumes that this file is adigital imageand most likely is safe, yet when opened, it runs the executable on the client machine.[77]Viruses may be installed on removable media, such asflash drives. The drives may be left in a parking lot of a government building or other target, with the hopes that curious users will insert the drive into a computer. In a 2015 experiment, researchers at the University of Michigan found that 45–98 percent of users would plug in a flash drive of unknown origin.[78] The vast majority of viruses target systems runningMicrosoft Windows. This is due to Microsoft's large market share ofdesktop computerusers.[79]The diversity of software systems on a network limits the destructive potential of viruses and malware.[a]Open-sourceoperating systems such asLinuxallow users to choose from a variety ofdesktop environments, packaging tools, etc., which means that malicious code targeting any of these systems will only affect a subset of all users. Many Windows users are running the same set of applications, enabling viruses to rapidly spread among Microsoft Windows systems by targeting the same exploits on large numbers of hosts.[80][81][82][83] While Linux and Unix in general have always natively prevented normal users from making changes to theoperating systemenvironment without permission, Windows users are generally not prevented from making these changes, meaning that viruses can easily gain control of the entire system on Windows hosts. This difference has continued partly due to the widespread use ofadministratoraccounts in contemporary versions likeWindows XP. In 1997, researchers created and released a virus for Linux—known as "Bliss".[84]Bliss, however, requires that the user run it explicitly, and it can only infect programs that the user has the access to modify. Unlike Windows users, most Unix users do notlog inas an administrator, or"root user", except to install or configure software; as a result, even if a user ran the virus, it could not harm their operating system. The Bliss virus never became widespread, and remains chiefly a research curiosity. Its creator later posted the source code toUsenet, allowing researchers to see how it worked.[85] Before computer networks became widespread, most viruses spread onremovable media, particularlyfloppy disks. In the early days of thepersonal computer, many users regularly exchanged information and programs on floppies. Some viruses spread by infecting programs stored on these disks, while others installed themselves into the diskboot sector, ensuring that they would be run when the user booted the computer from the disk, usually inadvertently. Personal computers of the era would attempt to boot first from a floppy if one had been left in the drive. Until floppy disks fell out of use, this was the most successful infection strategy and boot sector viruses were the most common in the "wild" for many years. Traditional computer viruses emerged in the 1980s, driven by the spread of personal computers and the resultant increase inbulletin board system(BBS),modemuse, and software sharing.Bulletin board–driven software sharing contributed directly to the spread ofTrojan horseprograms, and viruses were written to infect popularly traded software.Sharewareandbootlegsoftware were equally commonvectorsfor viruses on BBSs.[86][87]Viruses can increase their chances of spreading to other computers by infecting files on anetwork file systemor a file system that is accessed by other computers.[88] Macro viruseshave become common since the mid-1990s. Most of these viruses are written in the scripting languages for Microsoft programs such asMicrosoft WordandMicrosoft Exceland spread throughoutMicrosoft Officeby infecting documents andspreadsheets. Since Word and Excel were also available forMac OS, most could also spread toMacintosh computers. Although most of these viruses did not have the ability to send infectedemail messages, those viruses which did take advantage of theMicrosoft OutlookComponent Object Model(COM) interface.[89][90]Some old versions of Microsoft Word allow macros to replicate themselves with additional blank lines. If two macro viruses simultaneously infect a document, the combination of the two, if also self-replicating, can appear as a "mating" of the two and would likely be detected as a virus unique from the "parents".[91] A virus may also send aweb address linkas aninstant messageto all the contacts (e.g., friends and colleagues' e-mail addresses) stored on an infected machine. If the recipient, thinking the link is from a friend (a trusted source) follows the link to the website, the virus hosted at the site may be able to infect this new computer and continue propagating.[92]Viruses that spread usingcross-site scriptingwere first reported in 2002,[93]and were academically demonstrated in 2005.[94]There have been multiple instances of the cross-site scripting viruses in the "wild", exploiting websites such asMySpace(with the Samy worm) andYahoo!. In 1989 TheADAPSOSoftware Industry DivisionpublishedDealing With Electronic Vandalism,[95]in which they followed the risk of data loss by "the added risk of losing customer confidence."[96][97][98] Many users installantivirus softwarethat can detect and eliminate known viruses when the computer attempts todownloador run the executable file (which may be distributed as an email attachment, or onUSB flash drives, for example). Some antivirus software blocks known malicious websites that attempt to install malware. Antivirus software does not change the underlying capability of hosts to transmit viruses. Users must update their software regularly topatchsecurity vulnerabilities("holes"). Antivirus software also needs to be regularly updated to recognize the latestthreats. This is because malicioushackersand other individuals are always creating new viruses. The GermanAV-TESTInstitute publishes evaluations of antivirus software for Windows[99]and Android.[100] Examples of Microsoft Windowsanti virusand anti-malware software include the optionalMicrosoft Security Essentials[101](for Windows XP, Vista and Windows 7) for real-time protection, theWindows Malicious Software Removal Tool[102](now included withWindows (Security) Updateson "Patch Tuesday", the second Tuesday of each month), andWindows Defender(an optional download in the case of Windows XP).[103]Additionally, several capable antivirus software programs are available for free download from the Internet (usually restricted to non-commercial use).[104]Some such free programs are almost as good as commercial competitors.[105]Commonsecurity vulnerabilitiesare assignedCVE IDsand listed in the USNational Vulnerability Database.Secunia PSI[106]is an example of software, free for personal use, that will check a PC for vulnerable out-of-date software, and attempt to update it.Ransomwareandphishingscamalerts appear as press releases on theInternet Crime Complaint Center noticeboard. Ransomware is a virus that posts a message on the user's screen saying that the screen or system will remain locked or unusable until aransompayment is made.Phishingis a deception in which the malicious individual pretends to be a friend, computer security expert, or other benevolent individual, with the goal of convincing the targeted individual to revealpasswordsor other personal information. Other commonly used preventive measures include timely operating system updates, software updates, careful Internet browsing (avoiding shady websites), and installation of only trusted software.[107]Certain browsers flag sites that have been reported to Google and that have been confirmed as hosting malware by Google.[108][109] There are two common methods that an antivirus software application uses to detect viruses, as described in theantivirus softwarearticle. The first, and by far the most common method of virus detection is using a list ofvirus signaturedefinitions. This works by examining the content of the computer's memory (itsRandom Access Memory(RAM), andboot sectors) and the files stored on fixed or removable drives (hard drives, floppy drives, or USB flash drives), and comparing those files against adatabaseof known virus "signatures". Virus signatures are just strings of code that are used to identify individual viruses; for each virus, the antivirus designer tries to choose a unique signature string that will not be found in a legitimate program. Different antivirus programs use different "signatures" to identify viruses. The disadvantage of this detection method is that users are only protected from viruses that are detected by signatures in their most recent virus definition update, and not protected from new viruses (see "zero-day attack").[110] A second method to find viruses is to use aheuristicalgorithmbased on common virus behaviors. This method can detect new viruses for which antivirus security firms have yet to define a "signature", but it also gives rise to morefalse positivesthan using signatures. False positives can be disruptive, especially in a commercial environment, because it may lead to a company instructing staff not to use the company computer system until IT services have checked the system for viruses. This can slow down productivity for regular workers. One may reduce the damage done by viruses by making regularbackupsof data (and the operating systems) on different media, that are either kept unconnected to the system (most of the time, as in a hard drive),read-onlyor not accessible for other reasons, such as using differentfile systems. This way, if data is lost through a virus, one can start again using the backup (which will hopefully be recent).[111]If a backup session onoptical medialikeCDandDVDis closed, it becomes read-only and can no longer be affected by a virus (so long as a virus or infected file was not copied onto theCD/DVD). Likewise, an operating system on abootableCD can be used to start the computer if the installed operating systems become unusable. Backups on removable media must be carefully inspected before restoration. The Gammima virus, for example, propagates via removableflash drives.[112][113] Many websites run by antivirus software companies provide free online virus scanning, with limited "cleaning" facilities (after all, the purpose of the websites is to sell antivirus products and services). Some websites—likeGooglesubsidiaryVirusTotal.com—allow users to upload one or more suspicious files to be scanned and checked by one or more antivirus programs in one operation.[114][115]Additionally, several capable antivirus software programs are available for free download from the Internet (usually restricted to non-commercial use).[116]Microsoft offers an optional free antivirus utility calledMicrosoft Security Essentials, aWindows Malicious Software Removal Toolthat is updated as part of the regular Windows update regime, and an older optional anti-malware (malware removal) toolWindows Defenderthat has been upgraded to an antivirus product in Windows 8. Some viruses disableSystem Restoreand other important Windows tools such asTask ManagerandCMD. An example of a virus that does this is CiaDoor. Many such viruses can be removed byrebootingthe computer, entering Windows "safe mode" with networking, and then using system tools orMicrosoft Safety Scanner.[117]System RestoreonWindows Me,Windows XP,Windows VistaandWindows 7can restore theregistryand critical system files to a previous checkpoint. Often a virus will cause a system to "hang" or "freeze", and a subsequent hard reboot will render a system restore point from the same day corrupted. Restore points from previous days should work, provided the virus is not designed to corrupt the restore files and does not exist in previous restore points.[118][119] Microsoft'sSystem File Checker(improved in Windows 7 and later) can be used to check for, and repair, corrupted system files.[120]Restoring an earlier "clean" (virus-free) copy of the entire partition from acloned disk, adisk image, or abackupcopy is one solution—restoring an earlier backup disk "image" is relatively simple to do, usually removes any malware, and may be faster than "disinfecting" the computer—or reinstalling and reconfiguring the operating system and programs from scratch, as described below, then restoring user preferences.[111]Reinstalling the operating system is another approach to virus removal. It may be possible to recover copies of essential user data by booting from alive CD, or connecting the hard drive to another computer and booting from the second computer's operating system, taking great care not to infect that computer by executing any infected programs on the original drive. The original hard drive can then be reformatted and the OS and all programs installed from original media. Once the system has been restored, precautions must be taken to avoid reinfection from any restoredexecutable files.[121] The first known description of a self-reproducing program in fiction is in the 1970 short storyThe Scarred ManbyGregory Benfordwhich describes a computer program called VIRUS which, when installed on a computer withtelephone modemdialing capability, randomly dials phone numbers until it hits a modem that is answered by another computer, and then attempts to program the answering computer with its own program, so that the second computer will also begin dialing random numbers, in search of yet another computer to program. The program rapidly spreads exponentially through susceptible computers and can only be countered by a second program called VACCINE.[122]His story was based on an actual computer virus written inFORTRANthat Benford had created and run on thelabcomputer in the 1960s, as a proof-of-concept, and whichhe told John Brunner aboutin 1970.[123] The idea was explored further in two 1972 novels,When HARLIE Was OnebyDavid GerroldandThe Terminal ManbyMichael Crichton, and became a major theme of the 1975 novelThe Shockwave RiderbyJohn Brunner.[124] The 1973Michael Crichtonsci-fifilmWestworldmade an early mention of the concept of a computer virus, being a central plot theme that causesandroidsto run amok.[125][better source needed]Alan Oppenheimer's character summarizes the problem by stating that "...there's a clear pattern here which suggests an analogy to an infectious disease process, spreading from one...area to the next." To which the replies are stated: "Perhaps there are superficial similarities to disease" and, "I must confess I find it difficult to believe in a disease of machinery."[126] In 2016,Jussi Parikkaannounced the creation of The Malware Museum of Art: a collection of malware programs, usually viruses, distributed in the 1980s and 1990s on home computers. Malware Museum of Art is hosted atThe Internet Archiveand is curated byMikko HyppönenfromHelsinki,Finland.[127]The collection allows anyone with a computer to experience virus infection of decades ago with safety.[128] The term "virus" is also misused by extension to refer to other types ofmalware. "Malware" encompasses computer viruses along with many other forms of malicious software, such ascomputer "worms",ransomware,spyware,adware,trojan horses,keyloggers,rootkits,bootkits, maliciousBrowser Helper Object(BHOs), and other malicious software. The majority of active malware threats are trojan horse programs or computer worms rather than computer viruses. The term computer virus, coined byFred Cohenin 1985, is a misnomer.[129]Viruses often perform some type of harmful activity on infected host computers, such as acquisition ofhard diskspace orcentral processing unit(CPU) time, accessing and stealing private information (e.g.,credit cardnumbers,debit cardnumbers, phone numbers, names, email addresses, passwords, bank information, house addresses, etc.), corrupting data, displaying political, humorous or threatening messages on the user's screen,spammingtheir e-mail contacts,logging their keystrokes, or even rendering the computer useless. However, not all viruses carry a destructive "payload" and attempt to hide themselves—the defining characteristic of viruses is that they are self-replicating computer programs that modify other software without user consent by injecting themselves into the said programs, similar to a biological virus which replicates within living cells.	https://en.wikipedia.org/wiki/Computer_virus
Fault tree analysis(FTA) is a type offailure analysisin which an undesired state of a system is examined. This analysis method is mainly used insafety engineeringandreliability engineeringto understand how systems can fail, to identify the best ways to reduce risk and to determine (or get a feeling for) event rates of a safety accident or a particular system level (functional) failure. FTA is used in theaerospace,[1]nuclear power,chemical and process,[2][3][4]pharmaceutical,[5]petrochemicaland other high-hazard industries; but is also used in fields as diverse as risk factor identification relating tosocial servicesystem failure.[6]FTA is also used in software engineering for debugging purposes and is closely related to cause-elimination technique used to detect bugs. In aerospace, the more general term "system failure condition" is used for the "undesired state" / top event of the fault tree. These conditions are classified by the severity of their effects. The most severe conditions require the most extensive fault tree analysis. These system failure conditions and their classification are often previously determined in the functionalhazard analysis. Fault tree analysis can be used to:[7][8] Fault tree analysis (FTA) was originally developed in 1962 atBell Laboratoriesby H.A. Watson, under aU.S. Air ForceBallistics Systems Divisioncontract to evaluate theMinuteman IIntercontinental Ballistic Missile(ICBM) Launch Control System.[9][10][11][12]The use of fault trees has since gained widespread support and is often used as a failure analysis tool by reliability experts.[13]Following the first published use of FTA in the 1962 Minuteman I Launch Control Safety Study,BoeingandAVCOexpanded use of FTA to the entire Minuteman II system in 1963–1964. FTA received extensive coverage at a 1965System SafetySymposium inSeattlesponsored by Boeing and theUniversity of Washington.[14]Boeing began using FTA forcivil aircraftdesign around 1966.[15][16] Subsequently, within the U.S. military, application of FTA for use with fuses was explored byPicatinny Arsenalin the 1960s and 1970s.[17]In 1976 theU.S. Army Materiel Commandincorporated FTA into an Engineering Design Handbook on Design for Reliability.[18]The Reliability Analysis Center atRome Laboratoryand its successor organizations now with theDefense Technical Information Center(Reliability Information Analysis Center, and now Defense Systems Information Analysis Center[19]) has published documents on FTA and reliability block diagrams since the 1960s.[20][21][22]MIL-HDBK-338B provides a more recent reference.[23] In 1970, theU.S. Federal Aviation Administration(FAA) published a change to 14CFR25.1309airworthinessregulations fortransport categoryaircraftin theFederal Registerat 35 FR 5665 (1970-04-08). This change adopted failure probability criteria foraircraft systemsand equipment and led to widespread use of FTA in civil aviation. In 1998, the FAA published Order 8040.4,[24]establishing risk management policy including hazard analysis in a range of critical activities beyond aircraft certification, includingair traffic controland modernization of the U.S.National Airspace System. This led to the publication of the FAA System Safety Handbook, which describes the use of FTA in various types of formal hazard analysis.[25] Early in theApollo programthe question was asked about the probability of successfully sending astronauts to the moon and returning them safely to Earth. A risk, or reliability, calculation of some sort was performed and the result was a mission success probability that was unacceptably low. This result discouraged NASA from further quantitative risk or reliability analysis until after theChallengeraccident in 1986. Instead, NASA decided to rely on the use offailure modes and effects analysis (FMEA)and other qualitative methods for system safety assessments. After theChallengeraccident, the importance ofprobabilistic risk assessment(PRA) and FTA in systems risk and reliability analysis was realized and its use at NASA has begun to grow and now FTA is considered as one of the most important system reliability and safety analysis techniques.[26] Within the nuclear power industry, theU.S. Nuclear Regulatory Commissionbegan using PRA methods including FTA in 1975, and significantly expanded PRA research following the 1979 incident atThree Mile Island.[27]This eventually led to the 1981 publication of the NRC Fault Tree Handbook NUREG–0492,[28]and mandatory use of PRA under the NRC's regulatory authority. Following process industry disasters such as the 1984Bhopal disasterand 1988Piper Alphaexplosion, in 1992 theUnited States Department of LaborOccupational Safety and Health Administration(OSHA) published in the Federal Register at 57 FR 6356 (1992-02-24) itsProcess Safety Management(PSM) standard in 19 CFR 1910.119.[29]OSHA PSM recognizes FTA as an acceptable method forprocess hazard analysis(PHA). Today FTA is widely used insystem safetyandreliability engineering, and in all major fields of engineering. FTAmethodologyis described in several industry and government standards, including NRC NUREG–0492 for the nuclear power industry, an aerospace-oriented revision to NUREG–0492 for use byNASA,[26]SAEARP4761for civil aerospace, MIL–HDBK–338 for military systems,IECstandard IEC 61025[30]is intended for cross-industry use and has been adopted as European Norm EN 61025. Any sufficiently complex system is subject to failure as a result of one or more subsystems failing. The likelihood of failure, however, can often be reduced through improved system design. Fault tree analysis maps the relationship between faults, subsystems, and redundant safety design elements by creating a logic diagram of the overall system. The undesired outcome is taken as the root ('top event') of a tree of logic. For instance, the undesired outcome of a metal stamping press operation being considered might be a human appendage being stamped. Working backward from this top event it might be determined that there are two ways this could happen: during normal operation or during maintenance operation. This condition is a logical OR. Considering the branch of the hazard occurring during normal operation, perhaps it is determined that there are two ways this could happen: the press cycles and harms the operator, or the press cycles and harms another person. This is another logical OR. A design improvement can be made by requiring the operator to press two separate buttons to cycle the machine—this is a safety feature in the form of a logical AND. The button may have an intrinsic failure rate—this becomes a fault stimulus that can be analyzed. When fault trees are labeled with actual numbers for failure probabilities,computer programscan calculate failure probabilities from fault trees. When a specific event is found to have more than one effect event, i.e. it has impact on several subsystems, it is called a common cause or common mode. Graphically speaking, it means this event will appear at several locations in the tree. Common causes introduce dependency relations between events. The probability computations of a tree which contains some common causes are much more complicated than regular trees where all events are considered as independent. Not all software tools available on the market provide such capability. The tree is usually written out using conventionallogic gatesymbols. A cut set is a combination of events, typically component failures, causing the top event. If no event can be removed from a cut set without failing to cause the top event, then it is called a minimal cut set. Some industries use both fault trees andevent trees(seeProbabilistic Risk Assessment). An event tree starts from an undesired initiator (loss of critical supply, component failure etc.) and follows possible further system events through to a series of final consequences. As each new event is considered, a new node on the tree is added with a split of probabilities of taking either branch. The probabilities of a range of 'top events' arising from the initial event can then be seen. Classic programs include theElectric Power Research Institute's (EPRI) CAFTA software, which is used by many of the US nuclear power plants and by a majority of US and international aerospace manufacturers, and theIdaho National Laboratory'sSAPHIRE, which is used by the U.S. Government to evaluate the safety andreliabilityofnuclear reactors, theSpace Shuttle, and theInternational Space Station. Outside the US, the softwareRiskSpectrumis a popular tool for fault tree and event tree analysis, and is licensed for use at more than 60% of the world's nuclear power plants for probabilistic safety assessment. Professional-gradefree softwareis also widely available; SCRAM[31]is an open-source tool that implements the Open-PSA Model Exchange Format[32]open standard for probabilistic safety assessment applications. The basic symbols used in FTA are grouped as events, gates, and transfer symbols. Minor variations may be used in FTA software. Event symbols are used forprimary eventsandintermediate events. Primary events are not further developed on the fault tree. Intermediate events are found at the output of a gate. The event symbols are shown below: The primary event symbols are typically used as follows: An intermediate event gate can be used immediately above a primary event to provide more room to type the event description. FTA is a top-to-bottom approach. Gate symbols describe the relationship between input and output events. The symbols are derived from Boolean logic symbols: The gates work as follows: Transfer symbols are used to connect the inputs and outputs of related fault trees, such as the fault tree of a subsystem to its system. NASA prepared a complete document about FTA through practical incidents.[26] Events in a fault tree are associated withstatisticalprobabilitiesor Poisson-Exponentially distributed constant rates. For example, component failures may typically occur at some constantfailure rateλ (a constant hazard function). In this simplest case, failure probability depends on the rate λ and the exposure time t: P=1−e−λt{\displaystyle P=1-e^{-\lambda t}} where: P≈λt{\displaystyle P\approx \lambda t}ifλt<0.001{\displaystyle \lambda t<0.001} A fault tree is often normalized to a given time interval, such as a flight hour or an average mission time. Event probabilities depend on the relationship of the event hazard function to this interval. Unlike conventionallogic gatediagrams in which inputs and outputs hold thebinaryvalues of TRUE (1) or FALSE (0), the gates in a fault tree output probabilities related to theset operationsofBoolean logic. The probability of a gate's output event depends on the input event probabilities. An AND gate represents a combination ofindependentevents. That is, the probability of any input event to an AND gate is unaffected by any other input event to the same gate. Inset theoreticterms, this is equivalent to the intersection of the input event sets, and the probability of the AND gate output is given by: An OR gate, on the other hand, corresponds to set union: Since failure probabilities on fault trees tend to be small (less than .01), P (A ∩ B) usually becomes a very small error term, and the output of an OR gate may be conservatively approximated by using an assumption that the inputs aremutually exclusive events: An exclusive OR gate with two inputs represents the probability that one or the other input, but not both, occurs: Again, since P (A ∩ B) usually becomes a very small error term, the exclusive OR gate has limited value in a fault tree. Quite often, Poisson-Exponentially distributed rates[33]are used to quantify a fault tree instead of probabilities. Rates are often modeled as constant in time while probability is a function of time. Poisson-Exponential events are modelled as infinitely short so no two events can overlap. An OR gate is the superposition (addition of rates) of the two input failure frequencies or failure rates which are modeled asPoisson point processes. The output of an AND gate is calculated using the unavailability (Q1) of one event thinning the Poisson point process of the other event (λ2). The unavailability (Q2) of the other event then thins the Poisson point process of the first event (λ1). The two resulting Poisson point processes are superimposed according to the following equations. The output of an AND gate is the combination of independent input events 1 and 2 to the AND gate: In a fault tree, unavailability (Q) may be defined as the unavailability of safe operation and may not refer to the unavailability of the system operation depending on how the fault tree was structured. The input terms to the fault tree must be carefully defined. Many different approaches can be used to model a FTA, but the most common and popular way can be summarized in a few steps. A single fault tree is used to analyze one and only one undesired event, which may be subsequently fed into another fault tree as a basic event. Though the nature of the undesired event may vary dramatically, a FTA follows the same procedure for any undesired event; be it a delay of 0.25 ms for the generation of electrical power, an undetected cargo bay fire, or the random, unintended launch of anICBM. FTA analysis involves five steps: FTA is adeductive, top-down method aimed at analyzing the effects of initiating faults and events on a complex system. This contrasts withfailure mode and effects analysis(FMEA), which is aninductive, bottom-up analysis method aimed at analyzing the effects of single component or function failures on equipment or subsystems. FTA is very good at showing how resistant a system is to single or multiple initiating faults. It is not good at finding all possible initiating faults. FMEA is good at exhaustively cataloging initiating faults, and identifying their local effects. It is not good at examining multiple failures or their effects at a system level. FTA considers external events, FMEA does not.[35]In civil aerospace the usual practice is to perform both FTA and FMEA, with afailure mode effects summary(FMES) as the interface between FMEA and FTA. Alternatives to FTA includedependence diagram(DD), also known asreliability block diagram(RBD) andMarkov analysis. A dependence diagram is equivalent to a success tree analysis (STA), the logical inverse of an FTA, and depicts the system using paths instead of gates. DD and STA produce probability of success (i.e., avoiding a top event) rather than probability of a top event.	https://en.wikipedia.org/wiki/Fault_tree_analysis
Bot preventionrefers to the methods used by web services to prevent access byautomated processes. Studies suggest that over half of the traffic on the internet is bot activity, of which over half is further classified as 'bad bots'.[1] Bots are used for various purposes online. Some bots are used passively forweb scrapingpurposes, for example, to gather information fromairlinesabout flight prices and destinations. Other bots, such assneakerbots, help the bot operator acquire high-demand luxury goods; sometimes these areresoldon the secondary market at higher prices, in what is commonly known as 'scalping'.[2][3][4] Variousfingerprintingand behavioural techniques are used to identify whether theclientis a human user or a bot. In turn, bots use a range of techniques to avoid detection and appear like a human to the server.[2] Browser fingerprinting techniques are the most common component in anti-bot protection systems. Data is usually collected through client-sideJavaScriptwhich is then transmitted to the anti-bot service for analysis. The data collected includes results from JavaScript APIs (checking if a given API is implemented and returns the results expected from a normal browser), rendering complexWebGLscenes, andusing the Canvas API.[1][5]TLS fingerprintingtechniques categorise the client by analysing the supportedcipher suitesduring theSSL handshake.[6]These fingerprints can be used to createwhitelists/blacklistscontaining fingerprints of known browser stacks.[7]In 2017,Salesforceopen sourcedits TLS fingerprinting library (JA3).[8]Between August and September 2018, Akamai noticed a large increase in TLS tampering across its network to evade detection.[9][7] Behaviour-based techniques are also utilised, although less commonly than fingerprinting techniques, and rely on the idea that bots behave differently to human visitors. A common behavioural approach is to analyse a client'smouse movementsand determine if they are typical of a human.[1][10] More traditional techniques such asCAPTCHAsare also often employed, however they are generally considered ineffective while simultaneously obtrusive to human visitors.[11] The use of JavaScript can prevent some bots that rely on basic requests (such as viacURL), as these will not load the detection script and hence will fail to progress.[1]A common method to bypass many techniques is to use aheadless browserto simulate a realweb browserand execute the client-side JavaScript detection scripts.[2][1]There are a variety of headless browsers that are used; some are custom (such asPhantomJS) but it is also possible to operate typical browsers such asGoogle Chromein headless mode using a driver.Seleniumis a common web automation framework that makes it easier to control the headless browser.[5][1]Anti-bot detection systems attempt to identify the implementation of methods specific to these headless browsers, or the lack of proper implementation of APIs that would be implemented in regular web browsers.[1] The source code of these JavaScript files is typicallyobfuscatedto make it harder toreverse engineerhow the detection works.[5]Common techniques include:[12] Anti-bot protection services are offered by various internet companies, such asCloudflare[13]andAkamai.[14][15] In the United States, theBetter Online Tickets Sales Act(commonly known as the BOTS Act) was passed in 2016 to prevent some uses of bots in commerce.[16]A year later, the United Kingdom passed similar regulations in theDigital Economy Act 2017.[17][18]The effectiveness of these measures is disputed.[19]	https://en.wikipedia.org/wiki/Bot_prevention
Proof of personhood (PoP)is a means of resisting malicious attacks on peer to peer networks, particularly, attacks that utilize multiple fake identities, otherwise known as aSybil attack. Decentralized online platforms are particularly vulnerable to such attacks by their very nature, as notionally democratic and responsive to large voting blocks. In PoP, each unique human participant obtains one equal unit of voting power, and any associated rewards. The term is used in for cryptocurrency and blockchains as a parallel toproof of work,proof of stake, and otherconsensusmechanisms which attempt to distribute voting power and rewards to participants proportionately to an investment of resources. The problem ofSybil attacksusing many virtual identities has been recognized for decades as a fundamental challenge for distributed systems that expect each human user to have only one account or identity.[1]CAPTCHAsattempt to rate-limit automated Sybil attacks by using automatedTuring teststo distinguish humans from machines creating accounts or requesting services. Even when successful in this goal, however, CAPTCHAs allow one human to obtain multiple accounts or shares of a resource simply by solving multiple CAPTCHAs in succession, and thus do not satisfy the one-per-person goal in proof of personhood. Aside from CAPTCHAs allowing people to obtain multiple users, there are additional complications. Many users who are visually impaired or have learning disabilities may struggle to complete the puzzles. Additionally, some recently developed AI has succeeded in solving the CAPTCHA issue.[2] Distributed systems could require users to authenticate using strong identities verified by a government ortrusted third party, using anidentity verification serviceorself-sovereign identitysystem for example, but strong identification requirements conflict with theprivacyandanonymity, and increasebarriers to entry.[citation needed]One approach proposed to create anonymous but one-per-person credentials for use in distributed systems ispseudonym parties, in which participants gather periodically at in-person events and leverage the fact that humans can physically be in only one place at a time.[3] In 2014,Vitalik Buterinproposed the problem of creating a "unique identity system" for cryptocurrencies, which would give each human user one and only one anti-Sybil participation token.[4][non-primary source needed]In 2017, the term "proof of personhood" was proposed for an approach based on pseudonym parties.[5] A variety of approaches to implementing proof of personhood have been proposed, some in experimental deployment.[6] The approach originally proposed by Borge et al. was to use in-person pseudonym parties as a basis to create anonymous one-per-person tokens periodically without requiring any form of identity verification.[3][5]Theencointerproject adapts this approach by asking participants to meet in small groups simultaneously at randomly-chosen places, to verify each other's physical presence.[7] One drawback of this approach is the inconvenience to participants of going to designated physical locations at specific times, especially for participants with conflicting responsibilities at those times. Another issue is the challenge of organizingfederated pseudonym partiesin multiple locations simultaneously while allowing each group to verify that all other groups are organized honestly without inflating the number of digital credentials they issue.[citation needed] Another approach, related to thePGPWeb of Trust, relies on users forming asocial networkto verify and attest to each other's identities.[8]UniqueID incorporates biometric verification into the social network approach.[9] One criticism of the social network approach is that there is no straightforward way for a participant to verify that a social connection has not created other Sybil identities connected to and verified by other, disjoint sets of social contacts. A related challenge is that Sybil detection based on graph analysis make certain assumptions about the behavior of a Sybil attacker, and it is not clear that real-world social networks satisfy these assumptions.[10]Finally, graph-based Sybil detection algorithms tend to be able to detect only large, densely-clustered groups of Sybil nodes in a social network, leaving small-scale attacks difficult or impossible to distinguish by graph structure alone from legitimate users' connectivity structures.[citation needed] Another approach requires participants to have verified identities, but to hide oranonymizethose identities in subsequent use. One criticism of this approach is the privacy and surveillance risks inherent in such databases, especially biometric databases, and the level of trust users must place in the verification service for both Sybil protection and privacy of their identity information. Other critics highlight thatfacial recognition systemsfail on a global scale due to insufficient facial entropy.[citation needed] Apple, who are known for implementing a facial recognition feature into theiPhone, attempts to protect users' privacy with theSecure Enclave. The mathematical structure of a user's face captured by the TrueDepth camera does not leave the user's device, increasing the privacy and protection of personal information.[11][12]However, some concerns have been raised in regards to the level of security of the facial recognision on the devices. For example, there have been cases where family members were mistakenly recognized as their siblings.[13] Even with decentralized privacy protections, a criticism of this approach is the inconvenience and cost to users of verifying strong identities, and the risk of potentialexclusionof users who do not readily have or cannot afford the requisite identity documents, are reluctant to participate due to privacy and surveillance concerns, or are wrongly excluded by errors in biometric tests.[14] To resolve the security concerns over using biometrics to prove human uniqueness, only encrypting the biometrics data through cryptographic models is not enough. For this purpose,Humanodepresented a new technique to useConfidential computing,homomorphic encryptionalong withzero-knowledge proofto encrypt biometrics data in a way that the original biometrics data never leaves the device of the user. Instead, the decentralized network is provided only with the relevant information to verify if a person is a real human being through liveness detection.[citation needed] Another proposed class of approach extends theCAPTCHAprinciple of using Turing tests to the unique human verification problem. The Idena network, for example, assigns participants to verify each other usingfliptests.[15]Criticisms of this approach include the inconvenience to users of solving Turing tests, and whetherartificial intelligenceanddeepfaketechnologies will soon be able to solve such tests automatically or convince real participants that a synthetic user is human during a verification interaction.[citation needed] One proposed use for proof of personhood is to ensure that voting power in permissionlessconsensus algorithmsis widely distributed,[5]and to avoid the re-centralization that has been observed inproof of workmining pools,[16]and predicted inproof of stakesystems.[17] Another proposed use is to facilitatedemocraticgovernance in decentralized online systems, including blockchains and cryptocurrencies, that wish to enforce a "one person, one vote" rule.[18]	https://en.wikipedia.org/wiki/Proof_of_personhood
Proof of work(also written asproof-of-work, an abbreviatedPoW) is a form ofcryptographicproofin which one party (theprover) proves to others (theverifiers) that a certain amount of a specific computational effort has been expended.[1]Verifiers can subsequently confirm this expenditure with minimal effort on their part. The concept was first implemented inHashcashbyMoni NaorandCynthia Dworkin 1993 as a way to deterdenial-of-service attacksand other service abuses such asspamon a network by requiring some work from a service requester, usually meaning processing time by a computer. The term "proof of work" was first coined and formalized in a 1999 paper byMarkus Jakobssonand Ari Juels.[2][3]The concept was adapted to digital tokens byHal Finneyin 2004 through the idea of "reusable proof of work" using the 160-bit secure hash algorithm 1 (SHA-1).[4] Proof of work was later popularized byBitcoinas a foundation for consensus in a permissionless decentralized network, in which miners compete to append blocks and mine new currency, each miner experiencing a success probability proportional to the computational effort expended. PoW and PoS (proof of stake) remain the two best knownSybil deterrence mechanisms. In the context ofcryptocurrenciesthey are the most common mechanisms.[5] A key feature of proof-of-work schemes is their asymmetry: thework– the computation – must be moderately hard (yet feasible) on the prover or requester side but easy to check for the verifier or service provider. This idea is also known as a CPU cost function,client puzzle, computational puzzle, or CPU pricing function. Another common feature is built-inincentive-structures that reward allocating computational capacity to the network withvaluein the form of cryptocurrency.[6][7] The purpose of proof-of-work algorithms is not proving that certainworkwas carried out or that a computational puzzle was "solved", but deterring manipulation of data by establishing large energy and hardware-control requirements to be able to do so.[6]Proof-of-work systems have been criticized by environmentalists for their energy consumption.[8] The concept of Proof of Work (PoW) has its roots in early research on combating spam and preventing denial-of-service attacks. One of the earliest implementations of PoW wasHashcash, created by British cryptographerAdam Backin 1997.[9]It was designed as an anti-spam mechanism that required email senders to perform a small computational task, effectively proving that they expended resources (in the form of CPU time) before sending an email. This task was trivial for legitimate users but would impose a significant cost on spammers attempting to send bulk messages. Hashcash's system was based on the concept of finding a hash value that met certain criteria, a task that required computational effort and thus served as a "proof of work." The idea was that by making it computationally expensive to send large volumes of email,spammingwould be reduced. One popular system, used in Hashcash, uses partial hash inversions to prove that computation was done, as a goodwill token to send ane-mail. For instance, the following header represents about 252hash computations to send a message tocalvin@comics.neton January 19, 2038: It is verified with a single computation by checking that theSHA-1hash of the stamp (omit the header nameX-Hashcash:including the colon and any amount of whitespace following it up to the digit '1') begins with 52 binary zeros, that is 13 hexadecimal zeros:[1] Whether PoW systems can actually solve a particular denial-of-service issue such as the spam problem is subject to debate;[10][11]the system must make sending spam emails obtrusively unproductive for the spammer, but should also not prevent legitimate users from sending their messages. In other words, a genuine user should not encounter any difficulties when sending an email, but an email spammer would have to expend a considerable amount of computing power to send out many emails at once. Proof-of-work systems are being used by other, more complex cryptographic systems such as Bitcoin, which uses a system similar to Hashcash.[10] Proof of work traces its theoretical origins to early efforts to combat digital abuse, evolving significantly over time to address security, accessibility, and broader applications beyond its initial anti-spam purpose. The idea first emerged in 1993 as a deterrent for junk mail, but it wasSatoshi Nakamoto’s 2008 whitepaper, "Bitcoin: A Peer-to-Peer Electronic Cash System,"[12]that solidified proof of work's potential as a cornerstone of blockchain networks. This development reflects the rising demands for secure, trustless systems. The earliest appearance of proof of work was in 1993, whenCynthia DworkandMoni Naorproposed a system to curb junk email by requiring senders to perform computationally demanding tasks. In their paper, "Pricing via Processing or Combatting Junk Mail,"[13]they outlined methods such as computing modular square roots, designed to be challenging to solve yet straightforward to verify, establishing a foundational principle of proof of work’s asymmetry. This asymmetry is the crucial to the effectiveness of proof of work, ensuring that tasks like sending spam are costly for attackers, while verification remains efficient for legitimate users. This conceptual groundwork found practical use in 1997 withAdam Back’s Hashcash, a system that required senders to compute a partial hash inversion of theSHA-1algorithm, producing a hash with a set number of leading zeros. Described in Back’s paper "Hashcash: A Denial of Service Counter-Measure,"[14]Hashcash imposed a computational cost to deter spam while allowing recipients to confirm the work effortlessly, laying a critical foundation for subsequent proof of work implementations in cryptography and blockchain technology. Bitcoin, launched in 2009 by Satoshi Nakamoto, marked a pivotal shift by adapting Hashcash’s proof of work for cryptocurrency. Nakamoto’s Bitcoin whitepaper outlined a system using theSHA-256algorithm, where miners compete to solve cryptographic puzzles to append blocks to the blockchain, earning rewards in the process. Unlike Hashcash’s static proofs, Bitcoin’s proof of work algorithm dynamically adjusts its difficulty based on the time taken to mine the previous block, ensuring a consistent block time of approximately 10 minutes, creating a tamper-proof chain. This innovation transformed proof of work from a standalone deterrent into a consensus mechanism for a decentralized network, emphasizing financial incentives over computational effort. However, Bitcoin was not perfect. Miners began exploiting Bitcoin's proof of work with specialized hardware likeASICs. Initially mined with standardCPUs, Bitcoin saw a rapid transition toGPUsand then to ASIC, which vastly outperformed general hardware in solving SHA-256 puzzles. This gave ASICs miners an overwhelming advantage, rendering casual participants insignificant, which undermines Bitcoin's initial vision of a decentralized network accessible to all. To address Bitcoin’s increasing reliance on specialized hardware, proof of work evolved further with the introduction ofLitecoinin 2011, which adopted theScryptalgorithm. Developed byColin Percivaland detailed in the technical specification "The scrypt Password-Based Key Derivation Function,"[15]Scrypt was designed as a memory-intensive algorithm, requiring significant RAM to perform its computations. Unlike Bitcoin’s SHA-256, which favored powerful ASICs, Scrypt aimed to level the playing field by making mining more accessible to users with general-purpose hardware through heightened memory demands. However, over time, advancements in hardware led to the creation of Scrypt-specific ASICs, shifting the advantage back toward specialized hardware and reducing the algorithm's goal for decentralization. There are two classes of proof-of-work protocols. Known-solution protocols tend to have slightly lower variance than unbounded probabilistic protocols because the variance of arectangular distributionis lower than the variance of aPoisson distribution(with the same mean).[further explanation needed]A generic technique for reducing variance is to use multiple independent sub-challenges, as the average of multiple samples will have a lower variance. There are also fixed-cost functions such as the time-lock puzzle. Moreover, the underlying functions used by these schemes may be: Finally, some PoW systems offershortcutcomputations that allow participants who know a secret, typically a private key, to generate cheap PoWs. The rationale is that mailing-list holders may generate stamps for every recipient without incurring a high cost. Whether such a feature is desirable depends on the usage scenario. Here is a list of known proof-of-work functions: At theIACRconference Crypto 2022 researchers presented a paper describing Ofelimos, a blockchain protocol with aconsensus mechanismbased on "proof of useful work" (PoUW). Rather than miners consuming energy in solving complex, but essentially useless, puzzles to validate transactions, Ofelimos achieves consensus while simultaneously providing a decentralizedoptimization problem solver. The protocol is built around Doubly Parallel Local Search (DPLS), a local search algorithm that is used as the PoUW component. The paper gives an example that implements a variant ofWalkSAT, a local search algorithm to solve Boolean problems.[29] In 2009, the Bitcoin network went online. Bitcoin is a proof-of-work digital currency that, like Finney's RPoW, is also based on the Hashcash PoW. But in Bitcoin, double-spend protection is provided by a decentralized P2P protocol for tracking transfers of coins, rather than the hardware trusted computing function used by RPoW. Bitcoin has better trustworthiness because it is protected by computation. Bitcoins are "mined" using the Hashcash proof-of-work function by individual miners and verified by the decentralized nodes in the P2P Bitcoin network. The difficulty is periodically adjusted to keep theblock timearound a target time[30] Since the creation of Bitcoin, proof-of-work has been the predominant design ofpeer-to-peercryptocurrency. Studies have estimated the total energy consumption of cryptocurrency mining.[32]The PoW mechanism requires a vast amount of computing resources, which consume a significant amount of electricity. 2018 estimates from theUniversity of Cambridgeequate Bitcoin's energy consumption to that ofSwitzerland.[5] Each block that is added to the blockchain, starting with the block containing a given transaction, is called a confirmation of that transaction. Ideally, merchants and services that receive payment in the cryptocurrency should wait for at least one confirmation to be distributed over the network, before assuming that the payment was done. The more confirmations that the merchant waits for, the more difficult it is for an attacker to successfully reverse the transaction in a blockchain—unless the attacker controls more than half the total network power, in which case it is called a51% attack.[33] Within the Bitcoin community there are groups working together inmining pools.[34]Some miners useapplication-specific integrated circuits(ASICs) for PoW.[35]This trend toward mining pools and specialized ASICs has made mining some cryptocurrencies economically infeasible for most players without access to the latest ASICs, nearby sources of inexpensive energy, or other special advantages.[36] Some PoWs claim to be ASIC-resistant,[37]i.e. to limit the efficiency gain that an ASIC can have over commodity hardware, like a GPU, to be well under an order of magnitude. ASIC resistance has the advantage of keeping mining economically feasible on commodity hardware, but also contributes to the corresponding risk that an attacker can briefly rent access to a large amount of unspecialized commodity processing power to launch a51% attackagainst a cryptocurrency.[38] By design, Bitcoin's Proof of Work consensus algorithm is vulnerable to Majority Attacks (51% attacks). Any miner with over 51% of mining power is able to control the canonical chain until their hash power falls below 50%. This allows them to reorg the blockchain, double-spend, censor transactions, and completely control block production.[citation needed] Bitcoin has asymmetric security where Bitcoin miners control its security, but they aren't the same people who hold Bitcoin. Unlike with Proof of Stake, there is much weaker economic incentive for those who control security to protect the network under Proof of Work. Historically, many Proof of Work networks with low security budgets have fallen under 51% attacks.,[39]which highlights PoW's asymmetric security. The amount of protection provided by PoW mining is close to the security budget of the network, which is roughly equal to the total block reward. With each additional halving, Bitcoin's security budget continues to fall relative to its market cap. In the past, Bitcoin developers were hopeful that transaction fees would rise to replace the declining block subsidy, but this has not been the case as transaction fees still only generate 1% of the total block reward.[40]There are concerns that Bitcoin's security is unsustainable in the long run due to the declining security budget caused by its halvings. Miners compete to solve crypto challenges on the bitcoin blockchain, and their solutions must be agreed upon by all nodes and reach consensus. The solutions are then used to validate transactions, add blocks and generate new bitcoins. Miners are rewarded for solving these puzzles and successfully adding new blocks. However, the Bitcoin-style mining process is very energy intensive because the proof of work is shaped like a lottery mechanism. The underlying computational work has no other use but to provide security to the network that provides open access and has to work in adversarial conditions. Miners have to use a lot of energy to add a new block containing a transaction to the blockchain. The energy used in this competition is what fundamentally gives Bitcoin its level of security and resistance to attacks. Also, miners have to invest computer hardwares that need large spaces as fixed cost.[41] In January 2022 Vice-Chair of theEuropean Securities and Markets AuthorityErik Thedéen called on the EU to ban the proof of work model in favor of theproof of stakemodel due its lower energy emissions.[42] In November 2022 the state ofNew Yorkenacted a two-year moratorium on cryptocurrency mining that does not completely userenewable energyas a power source for two years. Existing mining companies will begrandfatheredin to continue mining without the use of renewable energy but they will not be allowed to expand or renew permits with the state. No new mining companies that do not completely use renewable energy will be allowed to begin mining.[43]	https://en.wikipedia.org/wiki/Proof_of_work
reCAPTCHAInc.[1]is aCAPTCHAsystem owned byGoogle. It enables web hosts to distinguish between human and automated access to websites. The original version asked users to decipher hard-to-read text or match images. Version 2 also asked users to decipher text or match images if the analysis of cookies and canvas rendering suggested the page was being downloaded automatically.[2]Since version 3, reCAPTCHA will never interrupt users and is intended to run automatically when users load pages or click buttons.[3] The original iteration of the service was amass collaborationplatform designed for the digitization of books, particularly those that were too illegible to bescanned by computers. The verification prompts utilized pairs of words from scanned pages, with one known word used as a control for verification, and the second used tocrowdsourcethe reading of an uncertain word.[4]reCAPTCHA was originally developed byLuis von Ahn, David Abraham,Manuel Blum, Michael Crawford, Ben Maurer, Colin McMillen, and Edison Tan atCarnegie Mellon University'smainPittsburghcampus.[5]It was acquired byGooglein September 2009.[6]The system helped to digitize the archives ofThe New York Times, and was subsequently used byGoogle Booksfor similar purposes.[7] The system was reported as displaying over 100 million CAPTCHAs every day,[8]on sites such asFacebook, TicketMaster, Twitter,4chan,CNN.com,StumbleUpon,[9]Craigslist(since June 2008),[10]and the U.S. National Telecommunications and Information Administration'sdigital TV converter boxcoupon program website (as part of theUS DTV transition).[11] In 2014, Google pivoted the service away from its original concept, with a focus on reducing the amount of user interaction needed to verify a user, and only presenting human recognition challenges (such as identifying images in a set that satisfy a specific prompt) if behavioral analysis suspects that the user may be a bot. In October 2023, it was found that OpenAI'sGPT-4chatbot could solve CAPTCHAs.[12]The service has been criticized for lack of security and accessibility while collecting user data, with a 2023 study estimating the collective cost of human time spent solving CAPTCHAs as $6.1 billion in wages.[13] Distributed Proofreaderswas the first project to volunteer its time to decipher scanned text that could not be read byoptical character recognition(OCR) programs. It works withProject Gutenbergto digitizepublic domainmaterial and uses methods quite different from reCAPTCHA. The reCAPTCHA program originated with Guatemalancomputer scientistLuis von Ahn,[14]and was aided by aMacArthur Fellowship. An early CAPTCHA developer, he realized "he had unwittingly created a system that was frittering away, in ten-second increments, millions of hours of a most precious resource: human brain cycles".[15] Scanned text is subjected to analysis by two different OCRs. Any word that is deciphered differently by the two OCR programs or that is not in an English dictionary is marked as "suspicious" and converted into a CAPTCHA. The suspicious word is displayed, out of context, sometimes along with a control word already known. If the human types the control word correctly, then the response to the questionable word is accepted as probably valid. If enough users were to correctly type the control word, but incorrectly type the second word which OCR had failed to recognize, then the digital version of documents could end up containing the incorrect word. The identification performed by each OCR program is given a value of 0.5 points, and each interpretation by a human is given a full point. Once a given identification hits 2.5 points, the word is considered valid. Those words that are consistently given a single identity by human judges are later recycled as control words.[16]If the first three guesses match each other but do not match either of the OCRs, they are considered a correct answer, and the word becomes a control word.[17]When six users reject a word before any correct spelling is chosen, the word is discarded as unreadable.[17] The original reCAPTCHA method was designed to show the questionable words separately, as out-of-context correction, rather than in use, such as within a phrase of five words from the original document.[18]Also, the control word might mislead the context for the second word, such as a request of "/metal/ /fife/" being entered as "metalfile" due to the logical connection of filing with a metal tool being considered more common than the musical instrument "fife".[citation needed] In 2012, reCAPTCHA began using photographs taken fromGoogle Street Viewproject, in addition to scanned words.[19]It will ask the user to identify images of crosswalks, street lights, and other objects. It has been hypothesized that the data is used byWaymo(a Google subsidiary) to train autonomous vehicles, though an unnamed representative has denied this, claiming the data was only being used to improve Google Maps as of mid-2021.[20] Google charges for the use of reCAPTCHA on websites that make over a million reCAPTCHA queries a month.[21] reCAPTCHA v1 was declaredend-of-lifeand shut down on March 31, 2018.[22] In 2013, reCAPTCHA began implementingbehavioral analysisof the browser's interactions to predict whether the user was a human or a bot. The following year, Google began to deploy a new reCAPTCHA API, featuring the "no CAPTCHA reCAPTCHA"—where users deemed to be of low risk only need to click a singlecheckboxto verify their identity. A CAPTCHA may still be presented if the system is uncertain of the user's risk; Google also introduced a new type of CAPTCHA challenge designed to be more accessible to mobile users, where the user must select images matching a specific prompt from a grid.[2][23] In 2017, Google introduced a new "invisible" reCAPTCHA, where verification occurs in the background, and no challenges are displayed at all if the user is deemed to be of low risk.[24][25][26]According to former Google "click fraudczar"Shuman Ghosemajumder, this capability "creates a new sort of challenge that very advanced bots can still get around, but introduces a lot less friction to the legitimate human."[26] The reCAPTCHA tests are displayed from the central site of the reCAPTCHA project, which supplies the words to be deciphered. This is done through aJavaScriptAPIwith the server making a callback to reCAPTCHA after the request has been submitted. The reCAPTCHA project provides libraries for various programming languages and applications to make this process easier. reCAPTCHA is a free-of-charge service provided to websites for assistance with the decipherment,[27]but the reCAPTCHA software is notopen-source.[28] Also, reCAPTCHA offers plugins for several web-application platforms includingASP.NET,Ruby, andPHP, to ease the implementation of the service.[29] The main purpose of aCAPTCHAsystem is to block spambots while allowing human users. On December 14, 2009, Jonathan Wilkins released a paper describing weaknesses in reCAPTCHA that allowed bots to achieve a solve rate of 18%.[31][32][33] On August 1, 2010, Chad Houck gave a presentation to theDEF CON18 Hacking Conference detailing a method to reverse the distortion added to images which allowed a computer program to determine a valid response 10% of the time.[34][35]The reCAPTCHA system was modified on July 21, 2010, before Houck was to speak on his method. Houck modified his method to what he described as an "easier" CAPTCHA to determine a valid response 31.8% of the time. Houck also mentioned security defenses in the system, including a high-security lockout if an invalid response is given 32 times in a row.[36] On May 26, 2012, Adam, C-P, and Jeffball of DC949 gave a presentation at the LayerOne hacker conference detailing how they were able to achieve an automated solution with an accuracy rate of 99.1%.[37]Their tactic was to use techniques from machine learning, a subfield of artificial intelligence, to analyze the audio version of reCAPTCHA which is available for the visually impaired. Google released a new version of reCAPTCHA just hours before their talk, making major changes to both the audio and visual versions of their service. In this release, the audio version was increased in length from 8 seconds to 30 seconds and is much more difficult to understand, both for humans as well as bots. In response to this update and the following one, the members of DC949 released two more versions of Stiltwalker which beat reCAPTCHA with an accuracy of 60.95% and 59.4% respectively. After each successive break, Google updated reCAPTCHA within a few days. According to DC949, they often reverted to features that had been previously hacked. On June 27, 2012, Claudia Cruz, Fernando Uceda, and Leobardo Reyes published a paper showing a system running on reCAPTCHA images with an accuracy of 82%.[38]The authors have not said if their system can solve recent reCAPTCHA images, although they claim their work to beintelligent OCRand robust to some, if not all changes in the image database. In an August 2012 presentation given at BsidesLV 2012, DC949 called the latest version "unfathomably impossible for humans"—they were not able to solve them manually either.[37]The web accessibility organization WebAIM reported in May 2012, "Over 90% of respondents [screen reader users] find CAPTCHA to be very or somewhat difficult".[39] The original iteration of reCAPTCHA was criticized as being a source ofunpaid workto assist in transcribing efforts.[40] Google profits from reCAPTCHA users as free workers to improve its AI research.[41] A 13-month study published in 2023, "Dazed & Confused: A Large-Scale Real-World User Study of reCAPTCHAv2," found that reCAPTCHA provides little security against bots and is primarily a tool to track user data, and has cost society an estimated 819 million hours of unpaid human labor.[42][13] The current iteration of the system has been criticized for its reliance ontracking cookiesand promotion ofvendor lock-inwith Google services; administrators are encouraged to include reCAPTCHA tracking code on all pages of their website to analyze the behavior and "risk" of users, which determines the level of friction presented when a reCAPTCHA prompt is used.[43]Google stated in itsprivacy policythat user data collected in this manner is not used for personalized advertising. It was also discovered that the system favors those who have an activeGoogle accountlogin, and displays a higher risk towards those using anonymizing proxies and VPN services.[24] Concerns were raised regarding privacy when Google announced reCAPTCHA v3.0, as it allows Google to track users on non-Google websites.[24] In April 2020,Cloudflareswitched from reCAPTCHA to hCaptcha, citing privacy concerns over Google's potential use of the data they recollect through reCAPTCHA fortargeted advertising[44]and to cut down on operating costs since a considerable portion of Cloudflare's customers are non-paying customers. In response, Google toldPC Magazinethat the data from reCAPTCHA is never used for personalized advertising purposes.[21] Google's help center states that reCAPTCHA is notsupportedfor thedeafblindcommunity,[45]effectively locking such users out of all pages that use the service. However, reCAPTCHA does currently have the longest list of accessibility considerations of any CAPTCHA service.[46] In one of the variants of CAPTCHA challenges, images are not incrementally highlighted, but fade out when clicked, and replaced with a new image fading in, resemblingwhack-a-mole. Criticism has been aimed at the long duration taken for the images to fade out and in.[47] reCAPTCHA also created the Mailhide project, which protectsemail addresseson web pages from beingharvestedbyspammers.[48]By default, the email address was converted into a format that did not allow acrawlerto see the full email address; for example, "mailme@example.com" would have been converted to "mai...@example.com". The visitor would then click on the "..." and solve the CAPTCHA to obtain the full email address. One could also edit the pop-up code so that none of the addresses were visible. Mailhide was discontinued in 2018 because it relied on reCAPTCHA v1.[49]	https://en.wikipedia.org/wiki/ReCAPTCHA
Antivirus software(abbreviated toAV software), also known asanti-malware, is acomputer programused to prevent, detect, and removemalware. Antivirus software was originally developed to detect and removecomputer viruses, hence the name. However, with the proliferation of othermalware, antivirus software started to protect against other computer threats. Some products also include protection from maliciousURLs,spam, andphishing.[1] The first known computer virus appeared in 1971 and was dubbed the "Creeper virus".[2]This computer virus infectedDigital Equipment Corporation's (DEC)PDP-10mainframe computers running theTENEXoperating system.[3][4] The Creeper virus was eventually deleted by a program created byRay Tomlinsonand known as "The Reaper".[5]Some people consider "The Reaper" the first antivirus software ever written – it may be the case, but it is important to note that the Reaper was actually a virus itself specifically designed to remove the Creeper virus.[5][6] The Creeper virus was followed by several other viruses. The first known that appeared "in the wild" was "Elk Cloner", in 1981, which infectedApple IIcomputers.[7][8][9] In 1983, the term"computer virus"was coined byFred Cohenin one of the first ever published academic papers oncomputer viruses.[10]Cohen used the term"computer virus"to describe programs that:"affect other computer programs by modifying them in such a way as to include a (possibly evolved) copy of itself."[11](note that a more recent definition ofcomputer virushas been given by the Hungarian security researcherPéter Szőr:"a code that recursively replicates a possibly evolved copy of itself").[12][13] The firstIBM PC compatible"in the wild" computer virus, and one of the first real widespread infections, was "Brain" in 1986. From then, the number of viruses has grown exponentially.[14][15]Most of the computer viruses written in the early and mid-1980s were limited to self-reproduction and had no specific damage routine built into the code. That changed when more and more programmers became acquainted with computer virus programming and created viruses that manipulated or even destroyed data on infected computers.[16] Beforeinternetconnectivity was widespread, computer viruses were typically spread by infectedfloppy disks. Antivirus software came into use, but was updated relatively infrequently. During this time, virus checkers essentially had to check executable files and the boot sectors of floppy disks and hard disks. However, as internet usage became common, viruses began to spread online.[17] There are competing claims for the innovator of the first antivirus product. Possibly, the first publicly documented removal of an "in the wild" computer virus (the "Vienna virus") was performed byBernd Fixin 1987.[18][19] In 1987, Andreas Lüning and Kai Figge, who foundedG Data Softwarein 1985, released their first antivirus product for theAtari STplatform.[20]In 1987, theUltimate Virus Killer (UVK)was also released.[21]This was the de facto industry standard virus killer for theAtari STandAtari Falcon, the last version of which (version 9.0) was released in April 2004.[citation needed]In 1987, in the United States,John McAfeefounded theMcAfeecompany and, at the end of that year, he released the first version ofVirusScan.[22]Also in 1987 (inCzechoslovakia), Peter Paško,Rudolf Hrubý, and Miroslav Trnka created the first version ofNODantivirus.[23][24] In 1987, Fred Cohen wrote thatthere is no algorithm that can perfectly detect all possible computer viruses.[25] Finally, at the end of 1987, the first twoheuristicantivirus utilities were released:Flushot PlusbyRoss Greenberg[26][27][28]andAnti4usby Erwin Lanting.[29]In hisO'Reillybook,Malicious Mobile Code: Virus Protection for Windows, Roger Grimes described Flushot Plus as "the first holistic program to fight malicious mobile code (MMC)."[30] However, the kind of heuristic used by early AV engines was totally different from those used today. The first product with a heuristic engine resembling modern ones wasF-PROTin 1991.[31]Early heuristic engines were based on dividing the binary into different sections: data section, code section (in a legitimate binary, it usually starts always from the same location). Indeed, the initial viruses re-organized the layout of the sections, or overrode the initial portion of a section in order to jump to the very end of the file where malicious code was located—only going back to resume execution of the original code. This was a very specific pattern, not used at the time by any legitimate software, which represented an elegant heuristic to catch suspicious code. Other kinds of more advanced heuristics were later added, such as suspicious section names, incorrect header size, regular expressions, and partial pattern in-memory matching. In 1988, the growth of antivirus companies continued. In Germany, Tjark Auerbach foundedAvira(H+BEDVat the time) and released the first version ofAntiVir(named"Luke Filewalker"at the time). InBulgaria, Vesselin Bontchev released his first freeware antivirus program (he later joinedFRISK Software). Also Frans Veldman released the first version ofThunderByte Antivirus, also known asTBAV(he sold his company toNorman Safegroundin 1998). InCzechoslovakia,Pavel Baudišand Eduard Kučera foundedAvast Software(at the timeALWIL Software) and released their first version of avast! antivirus. In June 1988, inSouth Korea,Ahn Cheol-Sooreleased its first antivirus software, calledV1(he founded AhnLab later in 1995). Finally, in autumn 1988, in the United Kingdom, Alan Solomon founded S&S International and created hisDr. Solomon's Anti-Virus Toolkit(although he launched it commercially only in 1991 – in 1998 Solomon's company was acquired byMcAfee, then known as Network Associates Inc.). Also in 1988, a mailing list named VIRUS-L[32]was started on theBITNET/EARNnetwork where new viruses and the possibilities of detecting and eliminating viruses were discussed. Some members of this mailing list were: Alan Solomon,Eugene Kaspersky(Kaspersky Lab),Friðrik Skúlason(FRISK Software),John McAfee(McAfee), Luis Corrons (Panda Security),Mikko Hyppönen(F-Secure),Péter Szőr, Tjark Auerbach (Avira) and Vesselin Bontchev (FRISK Software).[32] In 1989, inIceland,Friðrik Skúlasoncreated the first version ofF-PROT Anti-Virus(he foundedFRISK Softwareonly in 1993). Meanwhile, in the United States,Symantec(founded by Gary Hendrix in 1982) launched its firstSymantec antivirus for Macintosh(SAM).[33][34]SAM 2.0, released March 1990, incorporated technology allowing users to easily update SAM to intercept and eliminate new viruses, including many that didn't exist at the time of the program's release.[35] In the end of the 1980s, in United Kingdom, Jan Hruska and Peter Lammer founded the security firmSophosand began producing their first antivirus and encryption products. In the same period, in Hungary,VirusBusterwas founded (and subsequently incorporated bySophos).[36] In 1990, in Spain, Mikel Urizarbarrena foundedPanda Security(Panda Softwareat the time).[37]In Hungary, the security researcherPéter Szőrreleased the first version ofPasteurantivirus. In 1990, theComputer Antivirus Research Organization(CARO) was founded. In 1991, CARO released the"Virus Naming Scheme", originally written byFriðrik Skúlasonand Vesselin Bontchev.[38]Although this naming scheme is now outdated, it remains the only existing standard that most computer security companies and researchers ever attempted to adopt.CAROmembers includes: Alan Solomon, Costin Raiu, Dmitry Gryaznov,Eugene Kaspersky,Friðrik Skúlason,Igor Muttik,Mikko Hyppönen, Morton Swimmer, Nick FitzGerald,Padgett Peterson, Peter Ferrie, Righard Zwienenberg and Vesselin Bontchev.[39][40] In 1991, in the United States,Symantecreleased the first version ofNorton AntiVirus. In the same year, in theCzech Republic, Jan Gritzbach and Tomáš Hofer foundedAVG Technologies(Grisoftat the time), although they released the first version of theirAnti-Virus Guard(AVG) only in 1992. On the other hand, inFinland,F-Secure(founded in 1988 by Petri Allas and Risto Siilasmaa – with the name of Data Fellows) released the first version of their antivirus product.F-Secureclaims to be the first antivirus firm to establish a presence on the World Wide Web.[41] In 1991, theEuropean Institute for Computer Antivirus Research(EICAR) was founded to further antivirus research and improve development of antivirus software.[42][43] In 1992, in Russia, Igor Danilov released the first version ofSpiderWeb, which later becameDr.Web.[44] In 1994,AV-TESTreported that there were 28,613 unique malware samples (based on MD5) in their database.[45] Over time other companies were founded. In 1996, inRomania,Bitdefenderwas founded and released the first version ofAnti-Virus eXpert(AVX).[46]In 1997, in Russia,Eugene KasperskyandNatalya Kasperskyco-founded security firmKaspersky Lab.[47] In 1996, there was also the first "in the wild"Linuxvirus, known as"Staog".[48] In 1999,AV-TESTreported that there were 98,428 unique malware samples (based on MD5) in their database.[45] In 2000, Rainer Link and Howard Fuhs started the first open source antivirus engine, calledOpenAntivirus Project.[49] In 2001, Tomasz Kojm released the first version ofClamAV, the first ever open source antivirus engine to be commercialised. In 2007,ClamAVwas bought bySourcefire,[50]which in turn was acquired byCisco Systemsin 2013.[51] In 2002, in United Kingdom,Morten Lundand Theis Søndergaard co-founded the antivirus firm BullGuard.[52] In 2005,AV-TESTreported that there were 333,425 unique malware samples (based on MD5) in their database.[45] In 2007,AV-TESTreported a number of 5,490,960 new unique malware samples (based on MD5) only for that year.[45]In 2012 and 2013, antivirus firms reported a new malware samples range from 300,000 to over 500,000 per day.[53][54] Over the years it has become necessary for antivirus software to use several different strategies (e.g. specific email and network protection or low level modules) and detection algorithms, as well as to check an increasing variety of files, rather than just executables, for several reasons: In 2005,F-Securewas the first security firm that developed an Anti-Rootkit technology, calledBlackLight. Because most users are usually connected to the Internet on a continual basis,Jon Oberheidefirst proposed aCloud-basedantivirus design in 2008.[58] In February 2008 McAfee Labs added the industry-first cloud-based anti-malware functionality to VirusScan under the name Artemis. It was tested byAV-Comparativesin February 2008[59]and officially unveiled in August 2008 inMcAfee VirusScan.[60] Cloud AV created problems for comparative testing of security software – part of the AV definitions was out of testers control (on constantly updated AV company servers) thus making results non-repeatable. As a result,Anti-Malware Testing Standards Organisation(AMTSO) started working on method of testing cloud products which was adopted on May 7, 2009.[61] In 2011,AVGintroduced a similar cloud service, called Protective Cloud Technology.[62] Following the 2013 release of the APT 1 report fromMandiant, the industry has seen a shift towards signature-less approaches to the problem capable of detecting and mitigatingzero-day attacks.[63]Numerous approaches to address these new forms of threats have appeared, including behavioral detection, artificial intelligence, machine learning, and cloud-based file detection. According to Gartner, it is expected the rise of new entrants, suchCarbon Black,CylanceandCrowdstrikewill force end point protection incumbents into a new phase of innovation and acquisition.[64] One method fromBromiuminvolves micro-virtualization to protect desktops from malicious code execution initiated by the end user. Another approach fromSentinelOneandCarbon Blackfocuses on behavioral detection by building a full context around every process execution path in real time,[65][66]whileCylanceleverages an artificial intelligence model based on machine learning.[67] Increasingly, these signature-less approaches have been defined by the media and analyst firms as "next-generation" antivirus[68]and are seeing rapid market adoption as certified antivirus replacement technologies by firms such as Coalfire and DirectDefense.[69]In response, traditional antivirus vendors such asTrend Micro,[70]SymantecandSophos[71]have responded by incorporating "next-gen" offerings into their portfolios as analyst firms such asForresterandGartnerhave called traditional signature-based antivirus "ineffective" and "outdated".[72] As ofWindows 8, Windows includes its own free antivirus protection under theWindows Defenderbrand. Despite bad detection scores in its early days, AV-Test now certifies Defender as one of its top products.[73][74]While it isn't publicly known how the inclusion of antivirus software in Windows affected antivirus sales, Google search traffic for antivirus has declined significantly since 2010.[75]In 2014 Microsoft bought McAfee.[76] Since 2016, there has been a notable amount of consolidation in the industry.AvastpurchasedAVGin 2016 for $1.3 billion.[77]Avirawas acquired byNortonownerGen Digital(then NortonLifeLock) in 2020 for $360 million.[78]In 2021, theAviradivision ofGen Digitalacquired BullGuard.[79]The BullGuard brand was discontinued in 2022 and its customers were migrated to Norton. In 2022, Gen Digital acquired Avast, effectively consolidating four major antivirus brands under one owner.[80] In September 2024, following the US Commerce Department's ban onKaspersky, Pango Group acquired its customers (about 1 million).[81]The customers received continued services with no action required on their part. Then, in December 2024, Pango Group merged with Total Security, the provider of Total AV antivirus. The combined entity, now called Point Wild, has an enterprise value of $1.7 billion.[82] As of 2024, more than half of Americans use built-in antivirus protection for their devices likeMicrosoft DefenderorXProtectfromApple. However, about 121 million adults still use third-party antivirus software. Half of these adults use paid products, and about 50% of third-party software users - the owners of personal computers andWindowsoperating systems.[83]Antivirus programs on mobile devices are used by 17% of adults.[84] The 2025 antivirus market report confirms that most third-party antivirus users are on desktop devices, primarily aged between 35 and 45.[85]In contrast, younger users (18–25) tend to rely onad blockersinstead. In 1987,Frederick B. Cohendemonstrated that the algorithm which would be able to detect all possible viruses can't possibly exist (like the algorithm which determineswhether or not the given program halts).[25]However, using different layers of defense, a good detection rate may be achieved. There are several methods which antivirus engines can use to identify malware: Traditional antivirus software relies heavily uponsignaturesto identify malware.[92] Substantially, when a malware sample arrives in the hands of an antivirus firm, it is analysed by malware researchers or by dynamic analysis systems. Then, once it is determined to be a malware, a proper signature of the file is extracted and added to the signatures database of the antivirus software.[93] Although the signature-based approach can effectively contain malware outbreaks, malware authors have tried to stay a step ahead of such software by writing "oligomorphic", "polymorphic" and, more recently, "metamorphic" viruses, which encrypt parts of themselves or otherwise modify themselves as a method of disguise, so as to not match virus signatures in the dictionary.[94] Many viruses start as a single infection and through eithermutationor refinements by other attackers, can grow into dozens of slightly different strains, called variants. Generic detection refers to the detection and removal of multiple threats using a single virus definition.[95] For example, theVundotrojanhas several family members, depending on the antivirus vendor's classification.Symantecclassifies members of the Vundo family into two distinct categories,Trojan.VundoandTrojan.Vundo.B.[96][97] While it may be advantageous to identify a specific virus, it can be quicker to detect a virus family through a generic signature or through an inexact match to an existing signature. Virus researchers find common areas that all viruses in a family share uniquely and can thus create a single generic signature. These signatures often contain non-contiguous code, usingwildcard characterswhere differences lie. These wildcards allow the scanner to detect viruses even if they are padded with extra, meaningless code.[98]A detection that uses this method is said to be "heuristic detection". Anti-virus software can attempt to scan for rootkits. Arootkitis a type ofmalwaredesigned to gain administrative-level control over a computer system without being detected. Rootkits can change how theoperating systemfunctions and in some cases can tamper with the anti-virus program and render it ineffective. Rootkits are also difficult to remove, in some cases requiring a complete re-installation of the operating system.[99] Real-time protection, on-access scanning, background guard, resident shield, autoprotect, and other synonyms refer to the automatic protection provided by most antivirus, anti-spyware, and other anti-malware programs. This monitors computer systems for suspicious activity such as computer viruses, spyware, adware, and other malicious objects. Real-time protection detects threats in opened files and scans apps in real-time as they are installed on the device.[100]When inserting a CD, opening an email, or browsing the web, or when a file already on the computer is opened or executed.[101] Some commercial antivirus softwareend-user license agreementsinclude a clause that thesubscriptionwill be automatically renewed, and the purchaser's credit card automatically billed, at the renewal time without explicit approval. For example,McAfeerequires users to unsubscribe at least 60 days before the expiration of the present subscription,[102]whileBitdefendersends notifications to unsubscribe 30 days before the renewal.[103]Norton AntiVirusalso renews subscriptions automatically by default.[104] Some apparent antivirus programs are actuallymalwaremasquerading as legitimate software, such asWinFixer,MS Antivirus, andMac Defender.[105] A "false positive" or "false alarm" is when antivirus software identifies a non-malicious file as malware. When this happens, it can cause serious problems. For example, if an antivirus program is configured to immediately delete or quarantine infected files, as is common onMicrosoft Windowsantivirus applications, a false positive in an essential file can render the Windowsoperating systemor some applications unusable.[106]Recovering from such damage to critical software infrastructure incurs technical support costs and businesses can be forced to close whilst remedial action is undertaken.[107][108] Examples of serious false-positives: On the basis that Norton/Symantec has done this for every one of the last three releases of Pegasus Mail, we can only condemn this product as too flawed to use, and recommend in the strongest terms that our users cease using it in favour of alternative, less buggy anti-virus packages.[110] Running (the real-time protection of) multiple antivirus programs concurrently can degrade performance and create conflicts.[119]However, using a concept calledmultiscanning, several companies (includingG Data Software[120]andMicrosoft[121]) have created applications which can run multiple engines concurrently. It is sometimes necessary to temporarily disable virus protection when installing major updates such as Windows Service Packs or updating graphics card drivers.[122]Active antivirus protection may partially or completely prevent the installation of a major update. Anti-virus software can cause problems during the installation of an operating system upgrade, e.g. when upgrading to a newer version of Windows "in place"—without erasing the previous version of Windows. Microsoft recommends that anti-virus software be disabled to avoid conflicts with the upgrade installation process.[123][124][125]Active anti-virus software can also interfere with afirmwareupdate process.[126] The functionality of a few computer programs can be hampered by active anti-virus software. For example,TrueCrypt, a disk encryption program, states on its troubleshooting page that anti-virus programs can conflict with TrueCrypt and cause it to malfunction or operate very slowly.[127]Anti-virus software can impair the performance and stability of games running in theSteamplatform.[128] Support issues also exist around antivirus application interoperability with common solutions likeSSL VPN remote accessandnetwork access controlproducts.[129]These technology solutions often have policy assessment applications that require an up-to-date antivirus to be installed and running. If the antivirus application is not recognized by the policy assessment, whether because the antivirus application has been updated or because it is not part of the policy assessment library, the user will be unable to connect. Studies in December 2007 showed that the effectiveness of antivirus software had decreased in the previous year, particularly against unknown orzero day attacks. The computer magazinec'tfound that detection rates for these threats had dropped from 40 to 50% in 2006 to 20–30% in 2007. At that time, the only exception was theNOD32antivirus, which managed a detection rate of 68%.[130]According to theZeuS trackerwebsite the average detection rate for all variants of theZeuStrojan is as low as 40%.[131][independent source needed] The problem is magnified by the changing intent of virus authors. Some years ago it was obvious when a virus infection was present. At the time, viruses were written by amateurs and exhibited destructive behavior orpop-ups. Modern viruses are often written by professionals, financed bycriminal organizations.[132] In 2008,Eva Chen,CEOofTrend Micro, stated that the anti-virus industry has over-hyped how effective its products are—and so has been misleading customers—for years.[133] Independent testing on all the major virus scanners consistently shows that none provides 100% virus detection. The best ones provided as high as 99.9% detection for simulated real-world situations, while the lowest provided 91.1% in tests conducted in August 2013. Many virus scanners produce false positive results as well, identifying benign files as malware.[134] Although methods may differ, some notable independent quality testing agencies includeAV-Comparatives,ICSA Labs, SE Labs, West Coast Labs,Virus Bulletin,AV-TESTand other members of theAnti-Malware Testing Standards Organization.[135][136] Anti-virus programs are not always effective against new viruses, even those that use non-signature-based methods that should detect new viruses. The reason for this is that the virus designers test their new viruses on the major anti-virus applications to make sure that they are not detected before releasing them into the wild.[137] Some new viruses, particularlyransomware, usepolymorphic codeto avoid detection by virus scanners. Jerome Segura, a security analyst with ParetoLogic, explained:[138] It's something that they miss a lot of the time because this type of [ransomware virus] comes from sites that use a polymorphism, which means they basically randomize the file they send you and it gets by well-known antivirus products very easily. I've seen people firsthand getting infected, having all the pop-ups and yet they have antivirus software running and it's not detecting anything. It actually can be pretty hard to get rid of, as well, and you're never really sure if it's really gone. When we see something like that usually we advise to reinstall the operating system or reinstall backups.[138] Aproof of conceptvirus has used theGraphics Processing Unit(GPU) to avoid detection from anti-virus software. The potential success of this involves bypassing theCPUin order to make it much harder for security researchers to analyse the inner workings of such malware.[139] Detecting rootkits is a major challenge for anti-virus programs. Rootkits have full administrative access to the computer and are invisible to users and hidden from the list of running processes in thetask manager. Rootkits can modify the inner workings of theoperating systemand tamper with antivirus programs.[140] If a file has been infected by a computer virus, anti-virus software will attempt to remove the virus code from the file during disinfection, but it is not always able to restore the file to its undamaged state.[141][142]In such circumstances, damaged files can only be restored from existing backups orshadow copies(this is also true forransomware[143]); installed software that is damaged requires re-installation[144](however, seeSystem File Checker). Any writeable firmware in the computer can be infected by malicious code.[145]This is a major concern, as an infectedBIOScould require the actual BIOS chip to be replaced to ensure the malicious code is completely removed.[146]Anti-virus software is not effective at protecting firmware and themotherboardBIOS from infection.[147]In 2014, security researchers discovered thatUSBdevices contain writeable firmware which can be modified with malicious code (dubbed "BadUSB"), which anti-virus software cannot detect or prevent. The malicious code can run undetected on the computer and could even infect the operating system prior to it booting up.[148][149] Antivirus software has some drawbacks, first of which that it can impact acomputer's performance.[150] Furthermore, inexperienced users can be lulled into a false sense of security when using the computer, considering their computers to be invulnerable, and may have problems understanding the prompts and decisions that antivirus software presents them with. An incorrect decision may lead to a security breach. If the antivirus software employs heuristic detection, it must be fine-tuned to minimize misidentifying harmless software as malicious (false positive).[151] Antivirus software itself usually runs at the highly trustedkernellevel of theoperating systemto allow it access to all the potential malicious process and files, creating a potential avenue ofattack.[152]The USNational Security Agency(NSA) and the UKGovernment Communications Headquarters(GCHQ) intelligence agencies, respectively, have been exploiting anti-virus software to spy on users.[153]Anti-virus software has highly privileged and trusted access to the underlying operating system, which makes it a much more appealing target for remote attacks.[154]Additionally anti-virus software is "years behind security-conscious client-side applications like browsers or document readers. It means that Acrobat Reader, Microsoft Word or Google Chrome are harder to exploit than 90 percent of the anti-virus products out there", according to Joxean Koret, a researcher with Coseinc, a Singapore-basedinformation securityconsultancy.[154] Antivirus software running on individual computers is the most common method employed of guarding against malware, but it is not the only solution. Other solutions can also be employed by users, includingUnified Threat Management(UTM), hardware and network firewalls,Cloud-basedantivirus and online scanners. Network firewalls prevent unknown programs and processes from accessing the system. However, they are not antivirus systems and make no attempt to identify or remove anything. They may protect against infection from outside the protected computer ornetwork, and limit the activity of any malicious software which is present by blocking incoming or outgoing requests on certainTCP/IPports. Afirewallis designed to deal with broader system threats that come from network connections into the system and is not an alternative to a virus protection system. Cloud antivirus is a technology that uses lightweight agent software on the protected computer, while offloading the majority of data analysis to the provider's infrastructure.[155] One approach to implementing cloud antivirus involves scanning suspicious files using multiple antivirus engines. This approach was proposed by an early implementation of the cloud antivirus concept called CloudAV. CloudAV was designed to send programs or documents to anetwork cloudwhere multiple antivirus and behavioral detection programs are used simultaneously in order to improve detection rates. Parallel scanning of files using potentially incompatible antivirus scanners is achieved by spawning a virtual machine per detection engine and therefore eliminating any possible issues. CloudAV can also perform "retrospective detection", whereby the cloud detection engine rescans all files in its file access history when a new threat is identified thus improving new threat detection speed. Finally, CloudAV is a solution for effective virus scanning on devices that lack the computing power to perform the scans themselves.[156] Some examples of cloud anti-virus products arePanda Cloud AntivirusandImmunet.Comodo Grouphas also produced cloud-based anti-virus.[157][158] Some antivirus vendors maintain websites with free online scanning capability of the entire computer, critical areas only, local disks, folders or files. Periodic online scanning is a good idea for those that run antivirus applications on their computers because those applications are frequently slow to catch threats. One of the first things that malicious software does in an attack is disable any existing antivirus software and sometimes the only way to know of an attack is by turning to an online resource that is not installed on the infected computer.[159] Virus removal tools are available to help remove stubborn infections or a certain type of infection. Examples includeWindows Malicious Software Removal Tool,[160]KasperskyVirus Removal Tool,[161]andSophosScan & Clean.[162]It is also worth noting that sometimes antivirus software can produce a false-positive result, indicating an infection where there is none.[163] A rescue disk that is bootable, such as a CD or USB storage device, can be used to run antivirus software outside of the installed operating system in order to remove infections while they are dormant. A bootable rescue disk can be useful when, for example, the installed operating system is no longer bootable or has malware that is resisting all attempts to be removed by the installed antivirus software. Examples of software that can be used on a bootable rescue disk include theKasperskyRescue Disk,[164]Trend MicroRescue Disk,[165]andComodoRescue Disk.[166] According to an FBI survey, major businesses lose $12 million annually dealing with virus incidents.[167]A survey bySymantecin 2009 found that a third of small to medium-sized business did not use antivirus protection at that time, whereas more than 80% of home users had some kind of antivirus installed.[168]According to a sociological survey conducted byG Data Softwarein 2010 49% of women did not use any antivirus program at all.[169]	https://en.wikipedia.org/wiki/Antivirus_software
This is acomparison of firewalls. Based on theLinuxkernel Based on theLinuxkernel Based on theLinuxkernel. Based on theLinuxkernel Based on theLinuxkernel These are not strictly firewall features, but are sometimes bundled with firewall software or appliance. Features are also marked "yes" if an external module can be installed that meets the criteria.	https://en.wikipedia.org/wiki/Comparison_of_firewalls
ICSA Labs(International Computer Security Association) began as NCSA (National Computer Security Association). Its mission was to increase awareness of the need for computer security and to provide education about various security products and technologies. In its early days, NCSA focused almost solely on the certification ofanti-virus software. Using the Consortia model, NCSA worked together with anti-virus software vendors to develop one of the first anti-virus software certification schemes. Over the past decade, the organization added certification programs for other security-related products and changed its name to ICSA Labs. Operating as an independent division ofVerizon, ICSA Labs provides resources for research, intelligence, certification and testing of products, including anti-virus,firewall,IPsecVPN,cryptography, SSLVPN, networkIPS,anti-spywareand PC firewall products. ICSA Labstemporarily ceased operationin April 2017, restoring operations a year later. ICSA Labs ceased operation in 2022, following closure by its parent company Verizon. This in turn heralded the end ofThe WildList, a curated collection of computer virus samples, which ICSA Labs managed and distributed within the security industry for testing purposes. This article about a business, industry, or trade-related organization is astub. You can help Wikipedia byexpanding it.	https://en.wikipedia.org/wiki/International_Computer_Security_Association
Creating a unified list of computer viruses is challenging due to inconsistent naming conventions. To combat computer viruses and other malicious software, many security advisory organizations and anti-virus software developers compile and publish virus lists. When a new virus appears, the rush begins to identify and understand it as well as develop appropriate counter-measures to stop its propagation. Along the way, a name is attached to the virus. Since anti-virus software compete partly based on how quickly they react to the new threat, they usually study and name the viruses independently. By the time the virus is identified, many names have been used to denote the same virus. Ambiguity in virus naming arises when a newly identified virus is later found to be a variant of an existing one, often resulting in renaming. For example, the second variation of theSobigworm was initially called "Palyh" but later renamed "Sobig.b". Again, depending on how quickly this happens, the old name may persist. In terms of scope, there are two major variants: the list of "in-the-wild" viruses, which list viruses in active circulation, and lists of all known viruses, which also contain viruses believed not to be in active circulation (also called "zoo viruses"). The sizes are vastly different: in-the-wild lists contain a hundred viruses but full lists contain tens of thousands. Due to the continuous evolution of computer viruses and malware, virus naming conventions and classifications will continue to present challenges, making standardized virus databases essential for global cybersecurity.	https://en.wikipedia.org/wiki/Comparison_of_computer_viruses
Virus Bulletinis a magazine about the prevention, detection and removal ofmalwareandspam. It regularly features analyses of the latestvirusthreats, articles exploring new developments in the fight against viruses, interviews with anti-virus experts, and evaluations of currentanti-malwareproducts. Virus Bulletinwas founded in 1989[1]` as a monthly hardcopy magazine, and later distributed electronically in PDF format. The monthly publication format was discontinued in July 2014 and articles are now made available as standalone pieces on the website.[2]The magazine was originally located in theSophosheadquarters inAbingdon, Oxfordshirein the UK. It was co-founded and is owned by Jan Hruska and Peter Lammer, the co-founders of Sophos.Virus Bulletinclaims to have full editorial independence and not favour Sophos products in its tests and reviews.[3] Technical experts from anti-virus vendors have written articles for the magazine, which also conducts comparison tests of the detection rates of anti-virus software. Products which manage to detect 100% of the viruses in the wild, without false alarms, are given the VB100 award.[4] The magazine holds an annual conference (in late September or early October) for computer security professionals.[5]In recent years both magazine and conference have branched out to discuss anti-spam and other security issues as well as malware. Notable previous speakers includeMikko Hyppönen,[6]Eugene Kaspersky[7]andGraham Cluley, as well as representatives from all major anti-virus vendors.[8] Virus Bulletin was a founder member of theAnti-Malware Testing Standards Organizationand remains a member today. This computer magazine or journal-related article is astub. You can help Wikipedia byexpanding it. See tips for writing articles about magazines. Further suggestions might be found on the article'stalk page.	https://en.wikipedia.org/wiki/Virus_Bulletin
Anadvanced persistent threat(APT) is a stealthythreat actor, typically astateor state-sponsored group, which gains unauthorized access to acomputer networkand remains undetected for an extended period.[1][2]In recent times, the term may also refer to non-state-sponsored groups conducting large-scale targeted intrusions for specific goals.[3] Such threat actors' motivations are typically political or economic.[4]Every majorbusiness sectorhas recorded instances ofcyberattacksby advanced actors with specific goals, whether to steal, spy, or disrupt. These targeted sectors include government,defense,financial services,legal services,industrial,telecoms,consumer goodsand many more.[5][6][7]Some groups utilize traditionalespionagevectors, includingsocial engineering,human intelligenceandinfiltrationto gain access to a physical location to enable network attacks. The purpose of these attacks is to install custommalware.[8] APT attacks onmobile deviceshave also become a legitimate concern, since attackers are able to penetrate into cloud and mobile infrastructure to eavesdrop, steal, and tamper with data.[9] The median "dwell-time", the time an APT attack goes undetected, differs widely between regions.FireEyereported the mean dwell-time for 2018 in theAmericasas 71 days,EMEAas 177 days, andAPACas 204 days.[5]Such a long dwell-time allows attackers a significant amount of time to go through the attack cycle, propagate, and achieve their objectives. Definitions of precisely what an APT is can vary, but can be summarized by their named requirements below: Warnings against targeted, socially-engineered emails droppingtrojansto exfiltrate sensitive information were published by UK and USCERTorganisations in 2005. This method was used throughout the early 1990s and does not in itself constitute an APT. The term "advanced persistent threat" has been cited as originating from theUnited States Air Forcein 2006[13]with Colonel Greg Rattray cited as the individual who coined the term.[14] TheStuxnetcomputer worm, which targeted the computer hardware ofIran's nuclear program, is one example of an APT attack. In this case, the Iranian government might consider the Stuxnet creators to be an advanced persistent threat.[citation needed][15] Within thecomputer securitycommunity, and increasingly within the media, the term is almost always used in reference to a long-term pattern of sophisticated computer network exploitation aimed at governments, companies, and political activists, and by extension, also to ascribe the A, P and T attributes to the groups behind these attacks.[16]Advanced persistent threat (APT) as a term may be shifting focus to computer-based hacking due to the rising number of occurrences.PC Worldreported an 81 percent increase from 2010 to 2011 of particularly advanced targeted computer attacks.[17] Actors in many countries have usedcyberspaceas a means to gather intelligence on individuals and groups of individuals of interest.[18][19][20]TheUnited States Cyber Commandis tasked with coordinating the US military's offensive and defensivecyberoperations.[21] Numerous sources have alleged that some APT groups are affiliated with, or are agents of, governments ofsovereign states.[22][23][24]Businesses holding a large quantity ofpersonally identifiable informationare at high risk of being targeted by advanced persistent threats, including:[25] A Bell Canada study provided deep research into the anatomy of APTs and uncovered widespread presence in Canadian government and critical infrastructure. Attribution was established to Chinese and Russian actors.[28] Actors behind advanced persistent threats create a growing and changing risk to organizations' financial assets, intellectual property, and reputation[29]by following a continuous process orkill chain: In 2013, Mandiant presented results of their research on alleged Chinese attacks using APT method between 2004 and 2013[30]that followed similar lifecycle: In incidents analysed by Mandiant, the average period over which the attackers controlled the victim's network was one year, with longest – almost five years.[30]The infiltrations were allegedly performed by Shanghai-basedUnit 61398ofPeople's Liberation Army. Chinese officials have denied any involvement in these attacks.[32] Previous reports from Secdev had previously discovered and implicated Chinese actors.[33] There are tens of millions of malware variations,[34]which makes it extremely challenging to protect organizations from APT. While APT activities are stealthy and hard to detect, thecommand and controlnetwork traffic associated with APT can be detected at the network layer level with sophisticated methods. Deep log analyses and log correlation from various sources is of limited usefulness in detecting APT activities. It is challenging to separate noises from legitimate traffic. Traditional security technology and methods have been ineffective in detecting or mitigating APTs.[35]Active cyber defense has yielded greater efficacy in detecting and prosecuting APTs (find, fix, finish) when applyingcyber threat intelligenceto hunt and adversary pursuit activities.[36][37]Human-Introduced Cyber Vulnerabilities (HICV) are a weak cyber link that are neither well understood nor mitigated, constituting a significant attack vector.[38] Multiple organizations may assign different names to the same actor. As separate researchers could each have their own varying assessments of an APT group, companies such asCrowdStrike,Kaspersky,Mandiant, andMicrosoft, among others, have their own internal naming schemes.[82]Names between different organizations may refer to overlapping but ultimately different groups, based on various data gathered. CrowdStrike assigns animals by nation-state or other category, such as "Kitten" for Iran and "Spider" for groups focused on cybercrime.[83]Other companies have named groups based on this system — Rampant Kitten, for instance, was named by Check Point rather than CrowdStrike.[84] Dragos bases its names for APT groups on minerals.[82] Mandiant assigns numbered acronyms in three categories, APT, FIN, and UNC, resulting in APT names likeFIN7. Other companies using a similar system include Proofpoint (TA) and IBM (ITG and Hive).[82] Microsoft used to assign names from theperiodic table, often stylized in all-caps (e.g.POTASSIUM); in April 2023, Microsoft changed its naming schema to use weather-based names (e.g. Volt Typhoon).[85]	https://en.wikipedia.org/wiki/Advanced_persistent_threat
Deep content inspection(DCI) is a form of network filtering that examines an entire file orMIMEobject as it passes an inspection point, searching forviruses, spam, data loss, key words or other content level criteria. Deep Content Inspection is considered the evolution ofdeep packet inspectionwith the ability to look at what the actual content contains instead of focusing on individual or multiple packets. Deep content inspection allows services to keep track of content across multiple packets so that the signatures they may be searching for can cross packet boundaries and yet they will still be found. An exhaustive form of network traffic inspection in which Internet traffic is examined across all the sevenOSI ISO layers, and most importantly, the application layer.[1] Traditional inspection technologies are unable to keep up with the recent outbreaks of widespread attacks.[2]Unlike inspection methods such asdeep packet inspection(DPI), where only the data part (and possibly also the header) of a packet are inspected, deep content inspection (DCI)-based systems are exhaustive, such that network traffic packets are reassembled into their constituting objects, un-encoded and/or decompressed as required, and presented to be inspected for malware, right-of-use, compliance, and understanding of the traffic's intent. If this reconstruction and comprehension can be done in real-time, then real-time policies can be applied to traffic, preventing the propagation of malware, spam and valuable data loss. Further, with DCI, the correlation and comprehension of the digital objects transmitted in many communication sessions leads to new ways of network performance optimization and intelligence regardless of protocol or blended communication sessions. Historically, DPI was developed to detect and preventintrusion. It was then used to provideQuality of Servicewhere the flow of network traffic can be prioritized such that latency-sensitive traffic types (e.g., Voice over IP) can be utilized to provide higher flow priority. New generation of Network Content Security devices such as Unified Threat Management or Next Generation Firewalls (Garner RAS Core Research Note G00174908) use DPI to prevent attacks from a small percentage of viruses and worms; the signatures of these malware fit within the payload of a DPI's inspection scope. However, the detection and prevention of a new generation of malware such asConfickerandStuxnetis only possible through the exhaustive analysis provided by DCI.[3] Computer networks send information across a network from one point to another; the data (sometimes referred to as the payload) is ‘encapsulated’ within anIP packet, which looks as follows: *The IP Header provides address information - the sender and destination addresses, while the TCP/UDP Header provided other pertinent information such as the port number, etc. As networks evolve, inspection techniques evolve; all attempting to understand the payload. Throughout the last decade there have been vast improvements including: Historically, inspection technology examined only the IP Header and the TCP/UDP Header. Dubbed as ‘Packet Filtering’, these devices would drop sequence packets, or packets that are not allowed on a network. This scheme of network traffic inspection was first used by firewalls to protect against packet attacks. Stateful packet inspection was developed to examine header information and the packet content to increase source and destination understanding. Instead of letting the packets through as a result of their addresses and ports, packets stayed on the network if the context was appropriate to the networks’ current ‘state’. This scheme was first used by Check Point firewalls and eventually Intrusion Prevention/Detection Systems. Deep packet inspection is the predominant inspection tool used to analyze data packets passing through the network, including the headers and the data protocol structures. These technologies scan packet streams and look for offending patterns. To be effective, Deep Packet Inspection Systems must ‘string’ match Packet Payloads to malware signatures and specification signatures (which dictate what the request/response should be like) at wire speeds. To do so, FPGAs, or Field Programmable Gate Arrays, Network Processors, or even Graphics Processing Units (GPUs)[4]are programmed to be hardwired with these signatures and, as a result, traffic that passes through such circuitry is quickly matched. While using hardware allows for quick and inline matches, DPI systems have the following limitations including; Hardware limitations:Since DPI systems implement their pattern matching (or searches for ‘offending’ patterns) through hardware, these systems are typically limited by: Payload limitations:Web applications communicate content usingbinary-to-text encoding, compression (zipped, archived, etc.),obfuscationand evenencryption. As such payload structure is becoming more complex such that straight ‘string’ matching of the signatures is no longer sufficient. The common workaround is to have signatures be similarly ‘encoded’ or zipped which, given the above ‘search limitations’, cannot scale to support everyapplication type, ornested zipped or archived files. Parallel to the development of Deep Packet Inspection, the beginnings of Deep Content Inspection can be traced back as early as 1995 with the introduction of proxies that stopped malware or spam. Deep Content Inspection, can be seen as the third generation of Network Content Inspection, where network content is exhaustively examined, Proxies have been deployed to provide internet caching services to retrieve objects and then forward them. Consequently, all network traffic is intercepted, and potentially stored. These graduated to what is now known assecure web gateways, proxy-based inspections retrieve and scans object, script, and images. Proxies, which relies on a fetch the content first if it were not cached, then forwarding the content to the recipient introduced some form of file inspection as early as 1995 when MAILsweeper was released by Content Technologies (nowClearswift), which was then replaced by MIMEsweeper in 2005. 2006 saw the release of the open-source, cross-platform antivirus softwareClamAVprovided support for caching proxies,SquidandNetCache. Using theInternet Content Adaptation Protocol (ICAP), a proxy will pass the downloaded content for scanning to an ICAP server running an anti-virus software. Since complete files or ‘objects’ were passed for scanning, proxy-based anti-virus solutions are considered the first generation of network content inspection. BlueCoat, WebWasher and Secure Computing Inc. (now McAfee, now a division of Intel), provided commercial implementations of proxies, eventually becoming a standard network element in most enterprise networks. Limitations: While proxies (or secure web gateways) provide in-depth network traffic inspection, their use is limited as they: The Second generation of Network Traffic Inspection solutions were implemented in firewalls and/or UTMs. Given that network traffic is choked through these devices, in addition to DPI inspection, proxy-like inspection is possible. This approach was first pioneered byNetScreen Technologies Inc.(acquired byJuniper Networks Inc). However, given the expensive cost of such operation, this feature was applied in tandem with a DPI system and was only activated on a-per-need basis, or when content failed to be qualified through the DPI system. The third, and current, generation of network content inspection, known as deep content inspection solutions, are implemented as fully transparent devices that perform full application level content inspection at wire speed. In order to understand the communication session's intent —in its entirety—, a Deep Content Inspection System must scan both the handshake and payload. Once the digital objects (executables, images, JavaScript's, .pdfs, etc. also referred to as Data-In-Motion) carried within the payload are constructed, usability, compliance and threat analysis of this session and its payload can be achieved. Given that the handshake sequence and complete payload of the session is available to the DCI system, unlike DPI systems where simple pattern matching and reputation search are only possible, exhaustive object analysis is possible. The inspection provided by DCI systems can include signature matching, behavioral analysis, regulatory and compliance analysis, and correlation of the session under inspection to the history of previous sessions. Because of the availability of the complete payload's objects, and these schemes of inspection, deep content inspection systems are typically deployed where high-grade security and compliance is required or where end-point security solutions are not possible such as inbring your own device, or Cloud installations. This third generation approach of deep content inspection was developed within the defence and intelligence community, first appearing inguardproducts such as SyBard,[5]and later byWedge Networks Inc.. Key-implementation highlights of this Company's approach can be deduced from their patent USPTO# 7,630,379[6] The main differentiators of deep content inspection are: Deep content inspection is content-focused instead of analyzing packets or classifying traffic based on application types such as inNext Generation Firewalls. "Understanding" content and its intent is the highest level of intelligence to be gained from network traffic. This is important as information flow is moving away from packet, towards application, and ultimately to content. Example inspection levels: Because of the availability of the complete objects of that payload to a Deep Content Inspection system, some of the services/inspection examples can include: DCI is currently being adopted by enterprises, service providers and governments as a reaction to increasingly complex internet traffic with the benefits of understanding complete file types and their intent. Typically, these organizations have mission-critical applications with rigid requirements.[7] This type of inspection deals with real time protocols that only continue to increase in complexity and size. One of the key barriers for providing this level of inspection, that is looking at all content, is dealing with network throughput. Solutions must overcome this issue while not introducing latency into the network environment. They must also be able to effectively scale up to meet tomorrow's demands and the demands envisioned by the growing Cloud Computing trend. One approach is to use selective scanning; however, to avoid compromising accuracy, the selection criteria should be based on recurrence. The following patent USPTO# 7,630,379[8]provides a scheme as to how deep content inspection can be carried out effectively using a recurrence selection scheme. The novelty introduced by this patent is that it addresses issues such as content (E.g., an mp3 file) that could have been renamed before transmission. Dealing with the amount of traffic and information and then applying services requires very high speed look ups to be able to be effective. Need to compare against full services platforms or else having all traffic is not being utilized effectively. An example is often found in dealing with Viruses and Malicious content where solutions only compare content against a small virus database instead of a full and complete one.	https://en.wikipedia.org/wiki/Deep_content_inspection
Content Threat Removal(CTR) is acybersecuritytechnology intended to defeat the threat posed by handling digital content in the cyberspace.[1]Unlike other defenses, includingantivirus softwareandsandboxed execution, CTR does not rely on being able to detect threats. Similar toContent Disarm and Reconstruction, CTR is designed to remove the threat without knowing whether it has done so and acts without knowing if data contains a threat or not. Detection strategies work by detecting unsafe content, and then blocking or removing that content. Content that is deemed safe is delivered to its destination. In contrast, Content Threat Removal assumes all data is hostile and delivers none of it to the destination, regardless of whether it is actually hostile. Although no data is delivered, the business information carried by the data is delivered using new data created for the purpose. Advanced attacks continuously defeat defenses that are based on detection. These are often referred to aszero-dayattacks, because as soon as they are discovered attack detection mechanisms must be updated to identify and neutralize the attack, and until they are, all systems are unprotected. These attacks succeed because attackers find new ways of evading detection.Polymorphic codecan be used to evade the detection of known unsafe data and sandbox detection allows attacks to evade dynamic analysis.[2] A Content Threat Removal defence works by intercepting data on its way to its destination. The business information carried by the data is extracted and the data is discarded. Then entirely new, clean and safe data is built to carry the information to its destination. The effect of building new data to carry the business information is that any unsafe elements of the original data are left behind and discarded. This includes executable data, macros, scripts and malformed data that trigger vulnerabilities in applications. While CTR is a form of content transformation, not all transformations provide a complete defence against the content threat.[3] CTR is applicable to user-to-user traffic, such as email and chat, and machine-to-machine traffic, such as web services. Data transfers can be intercepted by in-line application layer proxies and these can transform the way information content is delivered to remove any threat.[4] CTR works by extracting business information from data and it is not possible to extract information from executable code. This means CTR is not directly applicable to web browsing, since most web pages are code. It can, however, be applied to content that is downloaded from, and uploaded to, websites. Although most web pages cannot be transformed to render them safe, web browsing can be isolated and the remote access protocols used to reach the isolated environment can be subjected to CTR. CTR provides a solution to the problem ofstegware.[5]It naturally removes detectable steganography and eliminates symbiotic and permutation steganography through normalisation.[6]	https://en.wikipedia.org/wiki/Content_Threat_Removal
In computing, thesame-origin policy(SOP) is a concept in the web-app application security model. Under the policy, a web browser permits scripts contained in a first web page to access data in a second web page, but only if both web pages have the sameorigin. An origin is defined as a combination of URI scheme, host name, and port number. This policy prevents a malicious script on one page from obtaining access to sensitive data on another web page through that page'sDocument Object Model (DOM). This mechanism bears a particular significance for modern web applications that extensively depend on HTTPS cookies to maintain authenticated user sessions, as servers act based on the HTTP cookie information to reveal sensitive information or perform state-changing actions. A strict separation between content provided by unrelated sites must be maintained on the client-side to prevent the loss of data confidentiality or integrity. The same-origin policy applies only to scripts. This means that resources such as images, CSS, and dynamically loaded scripts can be accessed across origins via the corresponding HTML tags (with fonts being a notable exception). Attacks take advantage of the fact that the same origin policy does not apply to HTML tags. There are some mechanisms available to relax the SOP, one of them isCross-Origin Resource Sharing(CORS). The concept of same-origin policy was introduced byNetscape Navigator 2.02in 1995,[1]shortly after the introduction ofJavaScriptin Netscape 2.0.[2][3]JavaScript enabledscriptingon web pages, and in particular programmatic access to the DOM. The policy was originally designed to protect access to the DOM, but has since been broadened to protect sensitive parts of the global JavaScript object. All modern browsers implement some form of the same-origin policy as it is an important security cornerstone.[4]The policies are not required to match an exact specification[5]but are often extended to define roughly compatible security boundaries for other web technologies, such asMicrosoft Silverlight,Adobe Flash, orAdobe Acrobat, or for mechanisms other than direct DOM manipulation, such asXMLHttpRequest. The algorithm used to calculate the "origin" of a URI is specified in RFC 6454, Section 4. For absolute URIs, the origin is the triple {scheme, host, port}. If the URI does not use a hierarchical element as a naming authority (seeRFC 3986, Section 3.2) or if the URI is not an absolute URI, then a globally unique identifier is used. Two resources are considered to be of the same origin if and only if all these values are exactly the same. To illustrate, the following table gives an overview of typical outcomes for checks against theURL"http://www.example.com/dir/page.html". Unlike other browsers, Internet Explorer does not include the port in the calculation of the origin, using the Security Zone in its place.[8] The same-origin policy protects against reusing authenticated sessions across origins. The following example illustrates a potential security risk that could arise without the same-origin policy. Assume that a user is visiting a banking website and doesn't log out. Then, the user goes to another site that has malicious JavaScript code that requests data from the banking site. Because the user is still logged in on the banking site, the malicious code could do anything the user could do on the banking site. For example, it could get a list of the user's last transactions, create a new transaction, etc. This is because, in the original spirit of a World Wide Web, browsers are required to tag along authentication details such as session cookies and platform-level kinds of the Authorization request header to the banking site based on the domain of the banking site. The bank site owners would expect that regular browsers of users visiting the malicious site do not allow the code loaded from the malicious site access the banking session cookie or platform-level authorization. While it is true that JavaScript has no direct access to the banking session cookie, it could still send and receive requests to the banking site with the banking site's session cookie. Same Origin Policy was introduced as a requirement for security-minded browsers to deny read access to responses from across origins, with the assumption that the majority of users choose to use compliant browsers. The policy does not deny writes. Counteracting the abuse of the write permission requires additionalCSRFprotections by the target sites. In some circumstances, the same-origin policy is too restrictive, posing problems for large websites that use multiplesubdomains. At first, a number of workarounds such as using thefragment identifieror thewindow.nameproperty were used to pass data between documents residing in different domains. Modern browsers support multiple techniques for relaxing the same-origin policy in a controlled manner: Netscape Navigatorbriefly contained ataint checkingfeature. The feature was experimentally introduced in 1997 as part of Netscape 3.[9]The feature was turned off by default, but if enabled by a user it would allow websites to attempt to read JavaScript properties of windows andframesbelonging to a different domain. The browser would then ask the user whether to permit the access in question.[10][11] If two windows (or frames) contain scripts that set domain to the same value, the same-origin policy is relaxed for these two windows, and each window can interact with the other. For example, cooperating scripts in documents loaded from orders.example.com and catalog.example.com might set theirdocument.domainproperties to “example.com”, thereby making the documents appear to have the same origin and enabling each document to read properties of the other. Setting this property implicitly sets the port to null, which most browsers will interpret differently from port 80 or even an unspecified port. To assure that access will be allowed by the browser, set the document.domain property of both pages.[12] Thedocument.domainconcept was introduced as part of Netscape Navigator 3,[13]released in 1996.[9] The other technique for relaxing the same-origin policy is standardized under the nameCross-Origin Resource Sharing(CORS). This standard extends HTTP with a new Origin request header and a new Access-Control-Allow-Origin response header.[14]It allows servers to use a header to explicitly list origins that may request a file or to use a wildcard and allow a file to be requested by any site. Browsers such as Firefox 3.5, Safari 4 and Internet Explorer 10 use this header to allow the cross-origin HTTP requests with XMLHttpRequest that would otherwise have been forbidden by the same-origin policy. Another technique,cross-document messagingallows a script from one page to pass textual messages to a script on another page regardless of the script origins. Calling the postMessage() method on a Window object asynchronously fires an "onmessage" event in that window, triggering any user-defined event handlers. A script in one page still cannot directly access methods or variables in the other page, but they can communicate safely through this message-passing technique. Since HTML<script>elements are allowed to retrieve and execute content from other domains, a page can bypass the same-origin policy and receive JSON data from a different domain by loading a resource that returns a JSONP payload. JSONP payloads consist of an internal JSON payload wrapped by a pre-defined function call. When the script resource is loaded by the browser, the designated callback function will be invoked to process the wrapped JSON payload. Modern browsers will permit a script to connect to a WebSocket address without applying the same-origin policy. However, they recognize when a WebSocket URI is used, and insert anOrigin:header into the request that indicates the origin of the script requesting the connection. To ensure cross-site security, the WebSocket server must compare the header data against an allowlist of origins permitted to receive a reply. The behavior of same-origin checks and related mechanisms is not well-defined in a number of corner cases such as for pseudo-protocols that do not have a clearly defined host name or port associated with their URLs (file:, data:, etc.). This historically caused a fair number of security problems, such as the generally undesirable ability of any locally stored HTML file to access all other files on the disk, or communicate with any site on the Internet. Lastly, certain types of attacks, such as DNS rebinding or server-side proxies, permit the host name check to be partly subverted, and make it possible for rogue web pages to directly interact with sites through addresses other than their "true", canonical origin. The impact of such attacks is limited to very specific scenarios, since the browser still believes that it is interacting with the attacker's site, and therefore does not disclose third-party cookies or other sensitive information to the attacker. Even when same-origin policy is in effect (without being relaxed by Cross-Origin Resource Sharing), certain cross-origin attacks can be performed.WebRTCcan be used to find out the internal IP address of a victim.[15]If attempting to connect to a cross-origin port, responses cannot be read in face of same-origin policy, but aJavaScriptcan still make inferences on whether the port is open or closed by checking if the onload/onerror event fires, or if we get a timeout. This gives opportunities for cross-originportscanning. Further, JavaScript snippets can use techniques like cross-site leaks[16]to exploit long-standing information leakages in the browser to infer information cross-origin. These attacks can be counteracted by implementing a Cross-Origin Resource Policy (CORP) header, which allows a website owner to block cross-origin or cross-site resources, like images, videos, and stylesheets. CORP can also block JavaScript-initiatedfetchrequests, but only if they are sent with theno-cors[17]request mode.[18] The same-origin policy does not prevent the browser from making GET, POST, OPTIONS, and TRACE requests; it only prevents the responses from being read by user code. Therefore, if an endpoint uses one of these "safe" request methods to write information or perform an action on a user's behalf, it can be exploited by attackers. Note that the Same-Origin Policy does not apply tocookiesfor historical reasons.[19]If multiple adversarial sites are deployed on the same hostname with different port numbers, contrary to the SOP, all cookies set by any of the sites are shared. This can be used to leak users' session tokens and steal account information. Therefore, web services should be separated by differentiatingsubdomainsrather than port numbers.	https://en.wikipedia.org/wiki/Same-origin_policy
NoScript(orNoScript Security Suite) is afree and open-sourceextensionforFirefox- andChromium-based web browsers,[4]written and maintained by Giorgio Maone,[5]a software developer and member of the Mozilla Security Group.[6] By default, NoScript blocks active (executable) web content, which can be wholly or partially unblocked by allowlisting a site or domain from the extension's toolbar menu or by clicking a placeholder icon. In the default configuration, active content is globally denied, although the user may turn this around and use NoScript to block specific unwanted content. The allowlist may be permanent or temporary (until the browser closes or the user revokes permissions). Active content may consist ofJavaScript, web fonts, mediacodecs,WebGL,Java applet,SilverlightandFlash. The add-on also offers specific countermeasures against security exploits.[7] Because many web browser attacks require active content that the browser normally runs without question, disabling such content by default and using it only to the degree that it is necessary reduces the chances of vulnerability exploitation. In addition, not loading this content saves significant bandwidth[8]and defeats some forms of web tracking. NoScript is useful for developers to see how well their site works with JavaScript turned off. It also can remove many irritating web elements, such as in-page pop-up messages and certainpaywalls, which require JavaScript in order to function. NoScript takes the form of atoolbaricon orstatus baricon in Firefox. It displays on every website to denote whether NoScript has either blocked, allowed, or partially allowed scripts to run on the web page being viewed. Clicking or hovering (since version 2.0.3rc1[9]) the mouse cursor on the NoScript icon gives the user the option to allow or forbid the script's processing. NoScript's interface, whether accessed by right-clicking on the web page or the distinctive NoScript box at the bottom of the page (by default), shows the URL of the script(s) that are blocked, but does not provide any sort of reference to look up whether or not a given script is safe to run.[10]With complex webpages, users may be faced with well over a dozen different cryptic URLs and a non-functioning webpage, with only the choice to allow the script, block the script or to allow it temporarily. On November 14, 2017, Giorgio Maone announced NoScript 10, which will be "very different" from 5.x versions, and will use WebExtension technology, making it compatible withFirefox Quantum.[11]On November 20, 2017, Maone released version 10.1.1 for Firefox 57 and above. NoScript is available for Firefox for Android.[12] On April 11, 2007, NoScript 1.1.4.7 was publicly released,[13]introducing the first client-side protection against Type 0 and Type 1cross-site scripting(XSS) ever delivered in a web browser. Whenever a website tries to inject HTML or JavaScript code inside a different site (a violation of thesame-origin policy), NoScript filters the malicious request and neutralizes its dangerous payload.[14] Similar features have been adopted years later byMicrosoft Internet Explorer 8[15]and byGoogle Chrome.[16] The Application Boundaries Enforcer (ABE) is a built-in NoScript module meant to harden theweb application-oriented protections already provided by NoScript, by delivering a firewall-like component running inside the browser. This "firewall" is specialized in defining and guarding the boundaries of each sensitive web application relevant to the user (e.g., plug-ins, webmail,online banking, and so on), according to policies defined directly by the user, the web developer/administrator, or a trusted third party.[17]In its default configuration, NoScript's ABE provides protection againstCSRFandDNS rebindingattacks aimed at intranet resources, such as routers and sensitive web applications.[18] NoScript's ClearClick feature,[19]released on October 8, 2008, prevents users from clicking on invisible or "redressed" page elements of embedded documents or applets, defeating all types ofclickjacking(i.e., from frames and plug-ins).[20] This makes NoScript "the only freely available product which offers a reasonable degree of protection against clickjacking attacks."[21] NoScript can force the browser to always useHTTPSwhen establishing connections to some sensitive sites, in order to prevent man-in-the-middle attacks. This behavior can be triggered either by the websites themselves, by sending theStrict Transport Securityheader, or configured by users for those websites that don't support Strict Transport Security yet.[22] NoScript's HTTPS enhancement features have been used by theElectronic Frontier Foundationas the basis of itsHTTPS Everywhereadd-on.[23] In May 2009, it was reported that an "extension war" had broken out between NoScript's developer, Giorgio Maone, and the developers of the Firefox ad-blocking extensionAdblock Plusafter Maone released a version of NoScript that circumvented a block enabled by an AdBlock Plus filter.[29][30]The code implementing this workaround was "camouflaged"[29]to avoid detection. Maone stated that he had implemented it in response to a filter that blocked his own website. After mounting criticism and a declaration by the administrators of theMozilla Add-onssite that the site would change its guidelines regarding add-on modifications,[31]Maone removed the code and issued a full apology.[29][32] In the immediate aftermath of the Adblock Plus incident,[33]a spat arose between Maone and the developers of theGhosteryadd-on after Maone implemented a change on his website that disabled the notification Ghostery used to reportweb tracking software.[34]This was interpreted as an attempt to "prevent Ghostery from reporting on trackers and ad networks on NoScript's websites".[33]In response, Maone stated that the change was made because Ghostery's notification obscured the donation button on the NoScript site.[35]This conflict was resolved when Maone changed his site's CSS to move—rather than disable—the Ghostery notification.[36]	https://en.wikipedia.org/wiki/NoScript
Mozilla Firefox, or simplyFirefox, is afree and open-source[12]web browserdeveloped by theMozilla Foundationand its subsidiary, theMozilla Corporation. It uses theGeckorendering engineto display web pages, which implements current and anticipated web standards.[13]Firefox is available forWindows 10or later versions ofWindows,macOS, andLinux.Its unofficial portsare available for variousUnixandUnix-likeoperating systems, includingFreeBSD,[14]OpenBSD,[15]NetBSD,[16]and other operating systems, such asreactOS. Firefox is also available forAndroidandiOS. However, as with all other iOS web browsers, the iOS version uses theWebKitlayout engine instead of Gecko due to platform requirements. An optimized version is also available on theAmazon Fire TVas one of the two main browsers available withAmazon's Silk Browser.[17] Firefox is thespiritual successorofNetscape Navigator, as theMozillacommunity was created byNetscapein 1998, before its acquisition byAOL.[18]Firefox was created in 2002 under the codename "Phoenix" by members of the Mozilla community who desired a standalone browser rather than theMozilla Application Suitebundle. During itsbetaphase, it proved to be popular with its testers and was praised for its speed, security, and add-ons compared toMicrosoft's then-dominantInternet Explorer 6. It was released on November 9, 2004,[19]and challengedInternet Explorer's dominance with 60 million downloads within nine months.[20]In November 2017, Firefox began incorporating new technology under the code name "Quantum" to promoteparallelismand a more intuitiveuser interface.[21] Firefox usage share grew to a peak of 32.21% in November 2009,[22]withFirefox 3.5overtakingInternet Explorer 7, although not all versions of Internet Explorer as a whole;[23][24]its usage then declined in competition withGoogle Chrome.[22]As of February 2025[update], according toStatCounter, it had a 6.36%usage shareon traditional PCs (i.e. as a desktop browser), making it the fourth-most popular PC web browser after Google Chrome (65%),Microsoft Edge(14%), andSafari(8.65%).[25] The project began as an experimental branch of theMozilla projectbyDave Hyatt,Joe Hewitt, andBlake Ross. They believed the commercial requirements ofNetscape's sponsorship and developer-drivenfeature creepcompromised the utility of the Mozilla browser.[26]To combat what they saw as theMozilla Suite'ssoftware bloat, they created a standalone browser, with which they intended to replace the Mozilla Suite.[27]Version 0.1 was released on September 23, 2002.[28]On April 3, 2003, theMozilla Organizationannounced that it planned to change its focus from the Mozilla Suite to Firefox andThunderbird.[29] The Firefox project has undergone several name changes.[30]The nascent browser was originally named Phoenix, after themythical birdthat rose triumphantly from the ashes of its dead predecessor (in this case, from the "ashes" ofNetscape Navigator, after it was sidelined by Microsoft Internet Explorer in the "First Browser War"). Phoenix was renamed in 2003 due to a trademark claim fromPhoenix Technologies. The replacement name, Firebird, provoked an intense response from theFirebirddatabase software project.[31][32]The Mozilla Foundation reassured them that the browser would always bear the name Mozilla Firebird to avoid confusion. After further pressure, Mozilla Firebird became Mozilla Firefox on February 9, 2004.[33]The name Firefox was said to be derived from a nickname of thered panda,[34]which became the mascot for the newly named project.[35]For the abbreviation of Firefox, Mozilla prefersFxorfx,although it is often abbreviated asFF[36]or Ff. The Firefox project went through many versions before version 1.0 and had already gained a great deal of acclaim from numerous media outlets, such asForbes[37]andThe Wall Street Journal.[38]Among Firefox's popular features were the integratedpop-up blocker,tabbed browsing, and an extension mechanism for adding functionality. Although these features have already been available for some time in other browsers such as theMozilla SuiteandOpera, Firefox was the first of these browsers to have achieved large-scale adoption so quickly.[39]Firefox attracted attention as an alternative toInternet Explorer, which had come under fire for its alleged poor program design and insecurity—detractors cite IE's lack of support for certain Web standards, use of the potentially dangerousActiveXcomponent, and vulnerability to spyware and malware installation.[citation needed]Microsoft responded by releasingWindows XPService Pack 2, which added several important security features to Internet Explorer 6.[40] Version 1.0 of Firefox was released on November 9, 2004.[41]This was followed by version 1.5 in November 2005, version 2.0 in October 2006, version 3.0 in June 2008, version 3.5 in June 2009, version 3.6 in January 2010, and version 4.0 in March 2011. From version 5 onwards, the development and release model changed into a "rapid" one; by the end of 2011 the stable release was version 9, and by the end of 2012 it reached version 17.[42] In 2016, Mozilla announced a project known asQuantum, which sought to improve Firefox's Gecko engine and other components to improve the browser's performance, modernize its architecture, and transition the browser to amulti-processmodel. These improvements came in the wake of decreasing market share toGoogle Chrome, as well as concerns that its performance was lapsing in comparison. Despite its improvements, these changes required existingadd-onsfor Firefox to be made incompatible with newer versions, in favor of a newextensionsystem that is designed to be similar to Chrome and other recent browsers. Firefox 57, which was released in November 2017, was the first version to contain enhancements from Quantum, and has thus been namedFirefox Quantum. A Mozilla executive stated that Quantum was the "biggest update" to the browser since version 1.0.[43][44][45]Unresponsive and crashing pages only affect other pages loaded within the same process. While Chrome uses separate processes for each loaded tab, Firefox distributes tabs over four processes by default (since Quantum), in order to balance memory consumption and performance. The process count can be adjusted, where more processes increase performance at the cost of memory, therefore suitable for computers with larger RAM capacity.[46][47] On May 3, 2019, the expiry of an intermediate signing certificate on Mozilla servers caused Firefox to automatically disable and lock all browser extensions (add-ons).[48][49]Mozilla began the roll-out of a fix shortly thereafter, using their Mozilla Studies component.[48][49] Support forAdobe Flashwas dropped on January 6, 2021, with the release of Firefox 85.[50] On June 1, 2021, Firefox's 'Proton' redesign was offered through its stable release channel[51]after being made available in the beta builds.[52]While users were initially allowed to revert to the old design throughabout:config, the correspondingkey-value pairsreportedly stopped working in later builds, resulting in criticism.[53]These included accessibility concerns[54][55]despite Mozilla's claim to "continue to work with the accessibility community"[56]and had not been resolved as of October 2024[update].[57] On January 13, 2022, an issue with Firefox's HTTP/3 implementation resulted in a widespread outage for several hours.[58] On September 26, 2023, Firefox 118.0 introduced on-device translation of web page content.[59] On January 23, 2024, along with the release of Firefox 122.0, Mozilla introduced an officialAPT repositoryforDebian-basedLinux distributions.[60] Features of the desktop edition includetabbed browsing, full-screen mode,spell checking,incremental search,smart bookmarks, bookmarking and downloading throughdrag and drop,[61][62]adownload manager,user profilemanagement,[63]private browsing, bookmark tags, bookmarkexporting,[64]offline mode,[65]ascreenshottool,web development tools, a "page info" feature which shows a list of page metadata and multimedia items,[66]a configuration menu atabout:configforpower users, and location-aware browsing (also known as "geolocation") based on a Google service.[67]Firefox has an integrated search system which uses Google by default in most markets.[68][69]DNS over HTTPSis another feature whose default behaviour is determined geographically.[70] Firefox provides an environment for web developers in which they can use built-in tools, such as the Error Console or theDOM Inspector, andextensions, such asFirebugand more recently there has been an integration feature withPocket. Firefox Hello was an implementation ofWebRTC, added in October 2014, which allows users of Firefox and other compatible systems to have a video call, with the extra feature of screen and file sharing by sending a link to each other. Firefox Hello was scheduled to be removed in September 2016.[71] Former features include aFile Transfer Protocol(FTP) client for browsing file servers, the ability to block images from individual domains (until version 72),[72]a3D page inspector(versions 11 to 46), tab grouping (until version 44), and the ability to add customized extra toolbars (until version 28).[73][74][75] Functions can be added throughadd-onscreated bythird-party developers. Add-ons are primarily coded using anHTML,CSS,JavaScript, withAPIknown asWebExtensions, which is designed to be compatible withGoogle ChromeandMicrosoft Edgeextension systems.[76]Firefox previously supported add-ons using theXULandXPCOMAPIs, which allowed them to directly access and manipulate much of the browser's internal functionality. As compatibility was not included in the multi-process architecture, XUL add-ons have been deemedLegacy add-onsand are no longer supported on Firefox 57 "Quantum" and newer.[77][78] Mozilla has occasionally installed extensions for users without their permission. This happened in 2017 when an extension designed to promote the showMr. Robotwas silently added in an update to Firefox.[79][80] Firefox can have themes added to it, which users can create or download from third parties to change the appearance of the browser.[81][82]Firefox also provides dark, light, and system themes. In 2013, Firefox for Android added aguest sessionmode, which wiped browsing data such as tabs, cookies, and history at the end of each guest session. Guest session data was kept even when restarting the browser or device, and deleted only upon a manual exit. The feature was removed in 2019, purportedly to "streamline the experience".[83][84] Firefox implements manyweb standards, includingHTML4(almost fullHTML5),XML,XHTML,MathML,SVG1.1 (full),[85]SVG 2 (partial),[86][87]CSS(with extensions),[88]ECMAScript (JavaScript),DOM,XSLT,XPath, andAPNG(AnimatedPNG) images withalpha transparency.[89]Firefox also implements standards proposals created by theWHATWGsuch as client-side storage,[90][91]and thecanvas element.[92]These standards are implemented through the Gecko layout engine, andSpiderMonkeyJavaScript engine. Firefox 4 was the first release to introduce significant HTML5 and CSS3 support. Firefox has passed theAcid2standards-compliance test since version 3.0.[93]Mozilla had originally stated that they did not intend for Firefox to pass theAcid3test fully because they believed that the SVG fonts part of the test had become outdated and irrelevant, due toWOFFbeing agreed upon as a standard by all major browser makers.[94]Because the SVG font tests were removed from the Acid3 test in September 2011, Firefox 4 and greater scored 100/100.[95][96] Firefox also implements "Safe Browsing,"[97]aproprietary protocol[98]from Google used to exchange data related with phishing and malware protection. Firefox supports the playback of video content protected byHTML5Encrypted Media Extensions(EME), since version 38. For security and privacy reasons, EME is implemented within a wrapper of open-source code that allows execution of aproprietaryDRMmodule byAdobe Systems—Adobe Primetime Content Decryption Module (CDM). CDM runs within a "sandbox" environment to limit its access to the system and provide it a randomized device ID to prevent services fromuniquely identifying the devicefor tracking purposes. The DRM module, once it has been downloaded, is enabled, and disabled in the same manner as otherplug-ins. Since version 47,[99]"Google's Widevine CDM on Windows and Mac OS X so streaming services likeAmazon Videocan switch fromSilverlightto encrypted HTML5 video" is also supported. Mozilla justified its partnership with Adobe and Google by stating: Firefox downloads and enables the Adobe Primetime and Google Widevine CDMs by default to give users a smooth experience on sites that require DRM. Each CDM runs in a separate container called a sandbox and you will be notified when a CDM is in use. You can also disable each CDM and opt-out of future updates and that it is "an important step on Mozilla's roadmap to removeNPAPIplugin support."[101]Upon the introduction of EME support, builds of Firefox on Windows were also introduced that exclude support for EME.[102][103]TheFree Software FoundationandCory Doctorowcondemned Mozilla's decision to support EME.[104] Firefox has been criticized by web developers for adopting web standard and fixing bugs which are decades old. No support for view transition, gradient and CSS features lack is also criticized.[105]Firefox scores less on bothHTML5 Testand JetStream2 compared to rival browsers.[106][107] Other issues include high battery usage, being highly resource intensive,[108]removal of tab group, use of telemetry, ads in search bar, dated download system, lack ofPWA,[109]and lack of ability to share text fragment.[110][111] From its inception, Firefox was positioned as a security-focused browser. At the time,Internet Explorer, the dominant browser, was facing a security crisis. Multiple vulnerabilities had been found, andmalwarelikeDownload.Jectcould be installed simply by visiting a compromised website. The situation was so bad that the US Government issued a warning against using Internet Explorer.[112]Firefox, being less integrated with the operating system, was considered a safer alternative since it was less likely to have issues that could completely compromise a computer. This led to a significant increase in Firefox's popularity during the early 2000s as a more secure alternative.[113][114]Moreover, Firefox was considered to have fewer actively exploitablesecurity vulnerabilitiescompared to its competitors. In 2006,The Washington Postreported that exploit code for known security vulnerabilities in Internet Explorer were available for 284 days compared to only nine days for Firefox before the problem was fixed.[115]ASymantecstudy around the same period showed that even though Firefox had a higher number of vulnerabilities, on average vulnerabilities were fixed faster in Firefox than in other browsers during that period.[116] During this period, Firefox used amonolithic architecture, like most browsers at the time. This meant all browser components ran in a singleprocesswith access to allsystem resources. This setup had multiple security issues. If a web page used too many resources, the entire Firefox process would hang or crash, affecting all tabs. Additionally, any exploit could easily access system resources, including user files. Between 2008 and 2012, most browsers shifted to a multiprocess architecture, isolating high-risk processes like rendering, media, GPU, and networking.[117]However, Firefox was slower to adopt this change. It wasn't until 2015 that Firefox started its Electrolysis (e10s) project to implement sandboxing across multiple components. This rewrite relied oninterprocess communicationusingChromium's interprocess communication library and placed various component including the rendering component in its own sandbox.[118]Firefox released this rewrite in to beta in August 2016, noting a 10–20% increase in memory usage, which was lower than Chrome's at the time.[119]However, the rewrite caused issues with their legacy extension API, which was not designed to work cross-process and requiredshim codeto function correctly.[119]After over a year in beta, the rewrite was enabled by default all users of Firefox in November 2017.[120] In 2012, Mozilla launched a new project calledServoto write a completely new and experimental browser engine utilizingmemory safetechniques written inRust.[121]In 2018, Mozilla opted to integrate parts of the Servo project into theGecko enginein a project codenamed the Quantum project.[122]The project completely overhauled Firefox's page rendering code resulting in performance and stability gains while also improving the security of existing components.[123]Additionally, the older incompatible extension API was removed in favour of a WebExtension API that more closely resembled Google Chrome's extension system. This broke compatibility with older extensions but resulted in fewer vulnerabilities and a much more maintainable extension system.[124]While the Servo project was intended to replace more parts of the Gecko Engine,[125]this plan never came to fruition. In 2020, Mozilla laid off all developers on the Servo team transferring ownership of the project to theLinux Foundation.[126] When Firefox initially released, it used a custom script permission policy where scripts that were signed by the page could gain access to higher privilege actions such as the ability to set a user's preferences. However, this model was not widely used and was later discontinued by Firefox. Modern day Firefox instead follows the standardsame-origin policypermission model that is followed by most modern browsers which disallows scripts from accessing any privileged data including data about other websites.[127] It usesTLSto protect communications with web servers using strongcryptographywhen using theHTTPSprotocol.[128]The freely availableHTTPS Everywhereadd-on enforces HTTPS, even if a regular HTTPURLis entered. Firefox now supports HTTP/2.[129] In February 2013, plans were announced for Firefox 22 to disablethird-party cookiesby default. However, the introduction of the feature was then delayed so Mozilla developers could "collect and analyze data on the effect of blocking some third-party cookies." Mozilla also collaborated withStanford University's "Cookie Clearinghouse" project to develop ablacklistandwhitelistof sites that will be used in the filter.[130][131] Version 23, released in August 2013, followed the lead of its competitors by blockingiframe, stylesheet, and script resources served from non-HTTPS servers embedded on HTTPS pages by default. Additionally,JavaScriptcould also no longer be disabled through Firefox's preferences, and JavaScript was automatically re-enabled for users who upgraded to 23 or higher with it disabled. The change was made due to the fact the JavaScript was being used across a majority of websites on the web and disabling JavaScript could potentially have untoward repercussions on inexperienced users who are unaware of its impact. Firefox also cited the fact that extensions likeNoScript, that can disable JavaScript in a more controlled fashion, were widely available. The following release added the ability to disable JavaScript through the developer tools for testing purposes.[132][133][134] Beginning with Firefox 48, all extensions must be signed by Mozilla to be used in release and beta versions of Firefox. Firefox 43 blocked unsigned extensions but allowed enforcement of extension signing to be disabled. All extensions must be submitted toMozilla Add-onsand be subject to code analysis in order to be signed, although extensions do not have to be listed on the service to be signed.[135][136]On May 2, 2019, Mozilla announced that it would be strengthening the signature enforcement with methods that included the retroactive disabling of old extensions now deemed to be insecure.[137] Since version 60 Firefox includes the option to useDNS over HTTPS(DoH), which causesDNS lookuprequests to be sent encrypted over the HTTPS protocol.[138][139]To use this feature the user must set certain preferences beginning with "network.trr" (Trusted Recursive Resolver) inabout:config: if network.trr.mode is 0, DoH is disabled; 1 activates DoH in addition to unencrypted DNS; 2 causes DoH to be used before unencrypted DNS; to use only DoH, the value must be 3. By setting network.trr.uri to the URL, specialCloudflareservers will be activated. Mozilla has a privacy agreement with this server host that restricts their collection of information about incoming DNS requests.[140] On May 21, 2019, Firefox was updated to include the ability to block scripts that used a computer'sCPUto minecryptocurrencywithout a user's permission, in Firefox version 67.0. The update also allowed users to block knownfingerprintingscripts that track their activity across the web, however it does not resist fingerprinting on its own.[141] In March 2021, Firefox launched SmartBlock in version 87 to offer protection againstcross-site tracking, without breaking the websites users visit.[142]Also known as state partitioning or "total cookie protection", SmartBlock works via a feature in the browser that isolates data from each site visited by the user to ensure that cross-site scripting is very difficult if not impossible. The feature also isolates local storage, service workers and other common ways for sites to store data.[143] In 2025, Mozilla introduced aterms of usefor Firefox, as a means to give more transparency over users' rights and permissions for the browser outside of the Mozilla Public License. The company received criticism centering around a clause that gave Mozilla a "nonexclusive, royalty-free, worldwide license" to use any information that was uploaded or inputted into the browser. The new terms were perceived to reduce privacy, and were seen to be connected to AI, while Mozilla denied that these were the motives.[144]Criticism centered on fears that the license grant covered all data inputted, while Mozilla responded saying that the change "does NOT give us ownership of your data".[145][146]In an attempt to respond to the fallout, Mozilla said that many modified words were to ease readability, increase transparency, formalize existing implicit agreements, and describe the circumstances of a free browser, adding that the AI features are covered by a separate agreement.[146][147]Days later, Mozilla changed the wording of their privacy FAQ,[148]removing a pledge to never "sell your personal data" and revising another section denying allegations that it sold user data, saying that it gathers some information from hideable advertisements as well as chatbot metadata when interacted with, and that the legal definition of "sell" was vague in some jurisdictions.[149][150] Firefox is a widelylocalizedweb browser. Mozilla uses the in-house Pontoon localization platform.[151]The first official release in November 2004 was available in 24 different languages and for 28locales.[152]In 2019, Mozilla released Project Fluent a localization system that allows translators to be more flexible with their translation than to be constrained in one-to-one translation of strings.[153][154]As of April 2025,[update]the supported versions of Firefox are available in 97 locales (88 languages).[9] There are desktop versions of Firefox for Microsoft Windows, macOS, and Linux, whileFirefox for Androidis available for Android (formerly Firefox for mobile, it also ran onMaemo,MeeGoandFirefox OS) andFirefox for iOSis available for iOS. Smartphones thatsupport Linuxbut not Android, or iOS apps can also run Firefox in its desktop version, for example usingpostmarketOS,MobianorUbuntu Touch.[155] Notes Firefox source code may becompiledfor various operating systems; however, officially distributed binaries are provided for the following: Firefox 1.0 was released forWindows 95, as well asWindows NT 4.0or later. Some users reported the 1.x builds were operable (but not installable) onWindows NT 3.51.[184] The version 42.0 release includes the firstx64build. It requiredWindows 7orServer 2008 R2.[185]Starting from version 49.0, Firefox for Windows requires and uses theSSE2instruction set. In September 2013, Mozilla released aMetro-style versionof Firefox, optimized fortouchscreenuse, on the "Aurora" release channel. However, on March 14, 2014, Mozilla cancelled the project because of a lack of user adoption.[186][187][188] In March 2017, Firefox 52 ESR, the last version of the browser forWindows XPandWindows Vista, was released.[189]Support for Firefox 52 ESR ended in June 2018.[190] Traditionally, installing the Windows version of Firefox entails visiting the Firefox website and downloading an installer package, depending on the desired localization and system architecture. In November 2021, Mozilla made Firefox available onMicrosoft Store. The Store-distributed package does not interfere with the traditional installation.[191][192] The last version of Firefox for Windows 7 and 8 is Firefox 115 ESR, which was released in July 2023.[193]Itsend-of-lifewas initially planned to be in October 2024,[194]however in July 2024, a Mozilla employee announced in a comment on Reddit that the company consider extending the support beyond the initial date, the duration of that extension being yet to be defined.[citation needed]In September 2024, the extension was announced for an initial period of six months.[195]In the release calendar page, a note states that Mozilla will re-evaluate the situation in early 2025 to see if another extension will be needed or not and statute about 115 ESR end-of-life then.[196]This extension has been renewed one more time, on February 18, 2025, for 6 additional months, which lead the end-of-life date on par with the 128 ESR branch, in September 2025.[197] The first official release (Firefox version 1.0) supportedmacOS(then called Mac OS X) on thePowerPCarchitecture. Mac OS X builds for theIA-32architecture became available via auniversal binarywhich debuted with Firefox 1.5.0.2 in 2006. Starting with version 4.0, Firefox was released for the x64 architecture to which macOS had migrated.[198]Version 4.0 also dropped support for PowerPC architecture, although other projects continued development of a PowerPC version of Firefox.[199] Firefox was originally released for Mac OS X 10.0 and higher.[200]The minimum OS then increased to Mac OS X 10.2 in Firefox 1.5 and 10.4 in Firefox 3.[201][202]Firefox 4 dropped support for Mac OS X 10.4 and PowerPC Macs, and Firefox 17 dropped support for Mac OS X 10.5 entirely.[203][204]The system requirements were left unchanged until 2016, when Firefox 49 dropped support for Mac OS X 10.6–10.8.[205][206]Mozilla ended support for OS X 10.9–10.11 in Firefox 79, with those users being supported on the Firefox 78 ESR branch until November 2021.[207][208][209]Most recently, Mozilla ended support formacOS 10.12–10.14in Firefox 116, with those users being supported on the Firefox 115 ESR branch until late 2024. In September 2024 however, an extension was announced for the 115 ESR branch for an initial period of six months.[195]This extension has been renewed one more time, leading the end-of-life date to September 2025[197] Since its inception, Firefox for Linux supported the 32-bit memory architecture of the IA-32 instruction set. 64-bit builds were introduced in the 4.0 release.[198]The 46.0 release replacedGTK2.18 with 3.4 as a system requirement on Linux and other systems runningX.Org.[210]Starting with 53.0, the 32-bit builds require theSSE2instruction set.[211] Firefox for mobile, code-named "Fennec", was first released forMaemoin January 2010 with version 1.0[212]and forAndroidin March 2011 with version 4.0.[213]Support for Maemo was discontinued after version 7, released in September 2011.[214]Fennec had a user interface optimized for phones and tablets. It included the Awesome Bar, tabbed browsing, add-on support, a password manager, location-aware browsing, and the ability to synchronize with the user's other devices with Mozilla Firefox usingFirefox Sync.[215]At the end of its existence, it had a market share of 0.5% on Android.[216] In August 2020, Mozilla launched a new version of its Firefox for Android app, named Firefox Daylight to the public[217]and codenamedFenix,[218]after a little over a year of testing.[219]It boasted higher speeds with its newGeckoViewengine, which is described as being "the only independentweb engine browseravailable onAndroid". It also added Enhanced Tracking Protection 2.0, a feature that blocks many knowntrackerson the Internet.[220]It also added the ability to place the address bar on the bottom, and a new Collections feature.[217]However, it was criticized for only having nineAdd-onsat launch, and missing certain features.[221][222][223]In response, Mozilla stated that they will allow more Add-ons with time.[224] Mozilla initially refused to port Firefox to iOS, due to the restrictions Apple imposed on third-party iOS browsers. Instead of releasing a full version of the Firefox browser, Mozilla released Firefox Home, a companion app for the iPhone and iPod Touch based on theFirefox Synctechnology, which allowed users to access their Firefox browsing history, bookmarks, and recent tabs. It also included Firefox's "Awesomebar" location bar. Firefox Home was not a web browser, the application launched web pages in either an embedded viewer for that one page, or by opening the page in the Safari app.[233][234]Mozilla pulled Firefox Home from theApp Storein September 2012, stating it would focus its resources on other projects. The company subsequently released thesource codeof Firefox Home's underlying synchronization software.[235] In April 2013, then-Mozilla CEOGary Kovacssaid that Firefox would not come to iOS if Apple required the use of theWebKitlayout engine to do so. One reason given by Mozilla was that prior to iOS 8, Apple had supplied third-party browsers with an inferior version of their JavaScript engine which hobbled their performance, making it impossible to match Safari's JavaScript performance on the iOS platform.[236]Apple later opened their "Nitro" JavaScript engine to third-party browsers.[237]In 2015, Mozilla announced it was moving forward with Firefox for iOS, with a preview release made available in New Zealand in September of that year.[238][239][240]It was fully released in November later that year.[241]It is the first Firefox-branded browser not to use theGeckolayout engineas is used in Firefox fordesktopandmobile. Apple's policies require all iOS apps that browse the web to use the built-inWebKitrendering framework and WebKit JavaScript, so using Gecko is not possible.[242][243]UnlikeFirefox on Android, Firefox for iOS does not support browser add-ons. In November 2016, Firefox released a new iOS app titledFirefox Focus, a private web browser.[244] Firefox Reality was released foraugmented realityandvirtual realityheadsets in September 2018.[245]It supports traditional web-browsing through 2D windows and immersive VR pages throughWeb VR. Firefox Reality is available onHTC Vive,Oculus,Google DaydreamandMicrosoft Hololensheadsets. In February 2022 Mozilla announced thatIgaliatook over stewardship of this project under the new name of Wolvic.[246] Firefox has also been ported toFreeBSD,[247]NetBSD,[248]OpenBSD,[249]OpenIndiana,[250]OS/2,[251]ArcaOS,[252]SkyOS,RISC OS[253]andBeOS/Haiku,[254][255][256][257]and an unofficial rebranded version calledTimberwolfhas been available forAmigaOS 4.[258] The Firefox port for OpenBSD is maintained by Landry Breuil since 2010. Firefox is regularly built for the current branch of the operating system, the latest versions are packaged for each release and remain frozen until the next release. In 2017, Landry began hosting packages of newer Firefox versions for OpenBSD releases from 6.0 onwards, making them available to installations without the ports system.[259] TheSolaris10 port of Firefox (includingOpenSolaris) was maintained by the Oracle Solaris Desktop Beijing Team,[260][261]until March 2018 when the team was disbanded. There was also an unofficial port ofFirefox 3.6.x toIBM AIX[262][263]and of v1.7.x toUnixWare.[264] In March 2011, Mozilla presented plans to switch to therapid release model, a faster 16-weekdevelopment cycle, similar toGoogle Chrome.Ars Technicanoted that this new cycle entailed "significant technical and operational challenges" for Mozilla (notably preserving third-partyadd-oncompatibility), but that it would help accelerate Firefox's adoption of new web standards, feature, and performance improvements.[265][266]This plan was implemented in April 2011.[267]The release process was split into four "channels", with major releases trickling down to the next channel every six to eight weeks. For example, the Nightly channel would feature a preliminary unstable version of Firefox 6, which would move to the experimental "Aurora" channel after preliminary testing, then to the more stable "beta" channel, before finally reaching the public release channel, with each stage taking around six weeks.[268][265][269]For corporations, Mozilla introduced an Extended Support Release (ESR) channel, with new versions released every 30 weeks (and supported for 12 more weeks after a new ESR version is released), though Mozilla warned that it would be less secure than the release channel, since security patches would only bebackportedfor high-impact vulnerabilities.[270][271] In 2017, Mozilla abandoned the Aurora channel, which saw low uptake, andrebasedFirefox Developer Edition onto the beta channel.[272]Mozilla usesA/B testing[273]and a staged rollout mechanism for the release channel, where updates are first presented to a small fraction of users, with Mozilla monitoring its telemetry for increased crashes or other issues before the update is made available to all users.[268]In 2020, Firefox moved to a four-week release cycle, to catch up with Chrome in support for new web features.[274][275]Chrome switched to a four-week cycle a year later.[276] Firefoxsource codeisfree software, with most of it being released under theMozilla Public License(MPL) version 2.0.[11]This license permits anyone to view, modify, or redistribute the source code. As a result, several publicly released applications have been built from it, including Firefox's predecessorNetscape,[277]the customizablePale Moon, and the privacy focusedTor Browser.[278] In the past, Firefox was licensed solely under the MPL, then version 1.1,[279]which theFree Software Foundationcriticized for beingweak copyleft, as the license permitted, in limited ways, proprietaryderivative works. Additionally, code only licensed under MPL 1.1 could not legally be linked with code under theGPL.[280][281]To address these concerns, Mozilla re-licensed most of Firefox under thetri-licensescheme of MPL 1.1, GPL 2.0, orLGPL2.1. Since the re-licensing, developers were free to choose the license under which they received most of the code, to suit their intended use: GPL or LGPL linking and derivative works when one of those licenses is chosen, or MPL use (including the possibility of proprietary derivative works) if they chose the MPL.[279]However, on January 3, 2012, Mozilla released the GPL-compatible MPL 2.0,[282]and with the release of Firefox 13 on June 5, 2012, Mozilla used it to replace the tri-licensing scheme.[283] The name "Mozilla Firefox" is aregistered trademarkof Mozilla; along with the official Firefox logo, it may only be used under certain terms and conditions. Anyone may redistribute the official binaries in unmodified form and use the Firefox name and branding for such distribution, but restrictions are placed on distributions which modify the underlying source code.[284]The name "Firefox" derives from a nickname of thered panda.[35]Mozilla celebrated Red Pandas.[285] Mozilla has placed the Firefox logo files under open-source licenses,[286][287]but its trademark guidelines do not allow displaying altered[288]or similar logos[289]in contexts where trademark law applies.[290] There has been some controversy over the Mozilla Foundation's intentions in stopping certain open-source distributions from using the "Firefox" trademark.[12]Open-source browsers "enable greater choice and innovation in the market rather than aiming for mass-market domination."[291]Mozilla Foundation ChairpersonMitchell Bakerexplained in an interview in 2007 that distributions could freely use the Firefox trademark if they did not modify source code, and that the Mozilla Foundation's only concern was with users getting a consistent experience when they used "Firefox".[292] To allow distributions of the codewithoutusing the official branding, the Firefoxbuild systemcontains a "branding switch". This switch, often used for alphas ("Auroras") of future Firefox versions, allows the code to be compiled without the official logo and name and can allow a derivative work unencumbered by restrictions on the Firefox trademark to be produced. In the unbranded build, the trademarked logo and name are replaced with a freely distributable generic globe logo and the name of the release series from which the modified version was derived.[citation needed] Distributing modified versions of Firefox under the "Firefox" name required explicit approval from Mozilla for the changes made to the underlying code, and required the use ofallof the official branding. For example, it was not permissible to use the name "Firefox" without also using the official logo. When theDebianproject decided to stop using the official Firefox logo in 2006 (because Mozilla's copyright restrictions at the time were incompatible withDebian's guidelines), they were told by a representative of the Mozilla Foundation that this was not acceptable and was asked either to comply with the published trademark guidelines or cease using the "Firefox" name in their distribution.[293]Debian switched to branding their modified version of Firefox "Iceweasel" (but in 2016 switched back to Firefox), along with other Mozilla software.GNU IceCatis another derived version of Firefox distributed by theGNU Project, which maintains its separate branding.[294] The Firefox icon is a trademark used to designate the official Mozilla build of the Firefox software and builds of official distribution partners.[295]For this reason, software distributors who distribute modified versions of Firefox do not use the icon.[290] Early Firebird and Phoenix releases of Firefox were considered to have reasonable visual designs but fell short when compared to many other professional software packages. In October 2003, professional interface designer Steven Garrity authored an article covering everything he considered to be wrong with Mozilla's visual identity.[296] Shortly afterwards, the Mozilla Foundation invited Garrity to head up the new visual identity team. The release of Firefox 0.8 in February 2004 saw the introduction of the new branding efforts. Included were new icon designs by silverorange, a group of web developers with a long-standing relationship with Mozilla. The final renderings are byJon Hicks, who had worked onCamino.[297][298]The logo was later revised and updated, fixing several flaws found when it was enlarged.[299]The animal shown in the logo is a stylized fox, although "firefox" is usually a common name for thered panda. The panda, according to Hicks, "didn't really conjure up the right imagery" and was not widely known.[298] In June 2019, Mozilla unveiled a revised Firefox logo, which was officially implemented on version 70. The new logo is part of an effort to build a brand system around Firefox and its complementary apps and services, which are now being promoted as a suite under the Firefox brand. Firefox was adopted rapidly, with 100 million downloads in its first year of availability.[302]This was followed by a series of aggressive marketing campaigns starting in 2004 with a series of eventsBlake Rossand Asa Dotzler called "marketing weeks".[303] Firefox continued to heavily market itself by releasing a marketing portal dubbed "Spread Firefox" (SFX) on September 12, 2004.[304]It debuted along with the Firefox Preview Release, creating a centralized space for the discussion of various marketing techniques. The release of theirmanifestostated that "the Mozilla project is a global community of people who believe that openness, innovation and opportunity are key to the continued health of the Internet."[291]A two-page ad in the edition of December 16 ofThe New York Times, placed by Mozilla Foundation in coordination with Spread Firefox, featured the names of the thousands of people worldwide who contributed to the Mozilla Foundation's fundraising campaign to support the launch of the Firefox 1.0 web browser.[305]SFX portal enhanced the "Get Firefox" button program, giving users "referrer points" as an incentive. The site lists the top 250 referrers. From time to time, the SFX team or SFX members launch marketing events organized at the Spread Firefox website. As a part of the Spread Firefox campaign, there was an attempt to break the world download record with the release of Firefox 3.[306]This resulted in an official certifiedGuinness world record, with over eight million downloads.[307]In February 2011, Mozilla announced that it would be retiring Spread Firefox (SFX). Three months later, in May 2011, Mozilla officially closed Spread Firefox. Mozilla wrote that "there are currently plans to create a new iteration of this website [Spread Firefox] at a later date."[308] In celebration of the third anniversary of the founding of theMozilla Foundation, the "World Firefox Day" campaign was established on July 15, 2006,[309][310]and ran until September 15, 2006.[311]Participants registered themselves and a friend on the website for nomination to have their names displayed on the Firefox Friends Wall, a digital wall that was displayed at the headquarters of the Mozilla Foundation. The Firefox community has also engaged in the promotion of their web browser. In 2006, some of Firefox's contributors fromOregon State Universitymade acrop circleof the Firefox logo in anoatfield nearAmity, Oregon, near the intersection of Lafayette Highway and Walnut Hill Road.[312]After Firefox reached 500 million downloads on February 21, 2008, the Firefox community celebrated by visitingFreericeto earn 500 million grains of rice.[313] Other initiatives included Live Chat – a service Mozilla launched in 2007 that allowed users to seek technical support from volunteers.[314]The service was later retired.[315] To promote the launch of Firefox Quantum in November 2017, Mozilla partnered withReggie Wattsto produce a series of TV ads and social media content.[316] In December 2005,Internet Weekran an article in which many readers reported high memory usage in Firefox 1.5.[317]Mozilla developers said that the higher memory use of Firefox 1.5 was at least partially due to the new fast backwards-and-forwards (FastBack) feature.[318]Other known causes of memory problems were malfunctioning extensions such asGoogle Toolbarand some older versions ofAdBlock,[319]or plug-ins, such as older versions of Adobe Acrobat Reader.[320]WhenPC Magazinein 2006 compared memory usage of Firefox 2,Opera 9, andInternet Explorer 7, they found that Firefox used approximately as much memory as each of the other two browsers.[321] In 2006,Softpedianoted that Firefox 1.5 took longer to start up than other browsers,[322]which was confirmed by furtherspeed tests.[323] Internet Explorer 6 launched more swiftly than Firefox 1.5 onWindows XPsince many of its components were built into the OS and loaded during system startup. As a workaround for the issue, a preloader application was created that loaded components of Firefox on startup, similar to Internet Explorer.[324]AWindows Vistafeature calledSuperFetchperforms a similar task of preloading Firefox if it is used often enough.[citation needed] Tests performed byPC Worldand Zimbra in 2006 indicated that Firefox 2 used less memory than Internet Explorer 7.[325][326]Firefox 3 used less memory than Internet Explorer 7, Opera 9.50 Beta,Safari3.1 Beta, and Firefox 2 in tests performed by Mozilla, CyberNet, and The Browser World.[327][328][329]In mid-2009, BetaNews benchmarked Firefox 3.5 and declared that it performed "nearly ten times better on XP than Microsoft Internet Explorer 7".[330] In January 2010, Lifehacker compared the performance of Firefox 3.5, Firefox 3.6, Google Chrome 4 (stable and Dev versions), Safari 4, and Opera (10.1 stable and 10.5 pre-alpha versions). Lifehacker timed how long browsers took to start and reach a page (both right after boot-up and after running at least once already), timed how long browsers took to load nine tabs at once, tested JavaScript speeds using Mozilla's Dromaeo online suite (which implements Apple'sSunSpiderand Google's V8 tests) and measured memory usage using Windows 7's process manager. They concluded that Firefox 3.5 and 3.6 were the fifth- and sixth-fastest browsers, respectively, on startup, 3.5 was third- and 3.6 was sixth-fastest to load nine tabs at once, 3.5 was sixth- and 3.6 was fifth-fastest on the JavaScript tests. They also concluded that Firefox 3.6 was the most efficient with memory usage followed by Firefox 3.5.[331] In February 2012,Tom's Hardwareperformance tested Chrome 17, Firefox 10,Internet Explorer 9, Opera 11.61, and Safari 5.1.2 on Windows 7.Tom's Hardwaresummarized their tests into four categories: Performance, Efficiency, Reliability, and Conformance. In the performance category they testedHTML5,Java,JavaScript,DOM,CSS 3,Flash,Silverlight, andWebGL(WebGL 2is current as of version 51; and Java and Silverlight stop working as of version 52)—they also tested startup time and page load time. The performance tests showed that Firefox was either "acceptable" or "strong" in most categories, winning three categories (HTML5, HTML5hardware acceleration, and Java) only finishing "weak" in CSS performance. In the efficiency tests,Tom's Hardwaretested memory usage and management. With this category, it determined that Firefox was only "acceptable" at performing light memory usage, while it was "strong" at performing heavy memory usage. In the reliability category, Firefox performed a "strong" amount of proper page loads. For the final category, conformance, it was determined that Firefox had "strong" conformance for JavaScript and HTML5. So in conclusion,Tom's Hardwaredetermined that Firefox was the best browser for Windows 7 OS, but that it only narrowly beat Google Chrome.[332] In June 2013,Tom's Hardwareagain performance tested Firefox 22, Chrome 27, Opera 12, andInternet Explorer 10. They found that Firefox slightly edged out the other browsers in their "performance" index, which examined wait times, JavaScript execution speed, HTML5/CSS3 rendering, and hardware acceleration performance. Firefox also scored the highest on the "non-performance" index, which measured memory efficiency, reliability, security, and standards conformance, finishing ahead of Chrome, the runner-up.Tom's Hardwareconcluded by declaring Firefox the "sound" winner of the performance benchmarks.[333] In January 2014, a benchmark testing the memory usage of Firefox 29, Google Chrome 34, andInternet Explorer 11indicated that Firefox used the least memory when a substantial number of tabs were open.[334] In benchmark testing in early 2015 on a "high-end" Windows machine, comparingMicrosoft Edge [Legacy], Internet Explorer, Firefox, Chrome, and Opera, Firefox achieved the highest score on three of the seven tests. Four different JavaScript performance tests gave conflicting results. Firefox surpassed all other browsers on thePeacekeeper benchmark, but was behind the Microsoft products when tested with SunSpider. Measured with Mozilla's Kraken, it came second place to Chrome, while on Google'sOctanechallenge it took third behind Chrome and Opera. Firefox took the lead with WebXPRT, which runs several typical HTML5 and JavaScript tasks. Firefox, Chrome, and Opera all achieved the highest possible score on the Oort Online test, measuring WebGL rendering speed (WebGL 2 is now current). In terms of HTML5 compatibility testing, Firefox was ranked in the middle of the group.[335] A similar set of benchmark tests in 2016 showed Firefox's JavaScript performance on Kraken and the newerJetstreamtests trailing slightly behind all other tested browsers except Internet Explorer (IE), which performed relatively poorly. On Octane, Firefox came ahead of IE and Safari, but again slightly behind the rest, includingVivaldiand Microsoft Edge [Legacy]. Edge [Legacy] took overall first place on the Jetstream and Octane benchmarks.[336] As of the adoption of Firefox 57 and Mozilla'sQuantum projectentering production browsers in November 2017, Firefox was tested to be faster than Chrome in independent JavaScript tests, and demonstrated to use less memory with many browser tabs opened.[337][338]TechRadarrated it as the fastest web browser in a May 2019 report.[339] Downloads have continued at an increasing rate since Firefox 1.0 was released, and as of 31 July 2009[update]Firefox had already been downloaded over one billion times.[342]This number does not include downloads using software updates or those from third-party websites.[343]They do not represent a user count, as one download may be installed on many machines, one person may download the software multiple times, or the software may be obtained from a third-party.[citation needed] In July 2010,IBMasked all employees (about 400,000) to use Firefox as their default browser.[344] Firefox was the second-most used web browser until November 2011, when Google Chrome surpassed it.[345]According to Mozilla, Firefox had more than 450 million users as of October 2012[update].[346][347] In October 2024, Firefox was the fourth-most widely used desktop browser, and it was the fourth-most popular with 2.95% of worldwideusage share of web browsersacross all platforms.[348] According to the Firefox Public Data report by Mozilla, the active monthly count of Desktop clients has decreased from around 310 million in 2017 to 200 million in 2023.[350]From Oct 2020, the desktop market share of Firefox started to decline in countries where it used to be the most popular. In Eritrea, it dropped from 50% in Oct 2020 to 9.32% in Sept 2021. In Cuba, it dropped from 54.36% in Sept 2020 to 38.42% in Sept 2021.[351][352] The UK[353]and US[354]governments both follow the 2% rule. This states that only browsers with more than 2% market share among visitors of their websites will be supported. There are concerns that support for Firefox will be dropped because as of December 29, 2023, the browser market share among US government website visitors is 2.2%.[355]	https://en.wikipedia.org/wiki/Firefox
uBlock Origin(/ˈjuːblɒk/YOO-blok[5]) is a free andopen-sourcebrowser extensionforcontent filtering, includingad blocking. The extension is available forFirefoxandChromium-based browsers (such asChrome,Edge,Brave, andOpera).[6] uBlock Origin is actively developed and maintained by its creator and lead developer Raymond Hill and the open source community.[1]As of April 2025[update], the Chrome version of uBlock Origin had over 29 million active users and the Firefox version had over 9 million active users, making it the most popular extension on Firefox.[7][8] The development of uBlock Origin (uBO) began byforkingthe codebase ofHTTP Switchboard, an extension designed to give users control over browser requests.uBlock, which was the predecessor of uBlock Origin, was further influenced byuMatrix, another browser extension created by forkingHTTP Switchboard.[9] HTTP Switchboardwas initially released on September 20, 2013.[10][11][12][13][14][15]It allowed users to control the types of requests made by their browser, providing a way to block specific types of web content. However, development ofHTTP Switchboardceased on May 18, 2015.[16][17] Following the end ofHTTP Switchboard’s development,uMatrixwas introduced on October 24, 2014.[18][9][19] This extension, designed for advanced users, acted as a request firewall, allowing users to control browser requests across two main dimensions: the domains and subdomains to which requests were sent, and the types of requests (such ascookies, images,XMLHttpRequest,frames, andscripts). uMatrix retained much of the user interface fromHTTP Switchboard, but significantly expanded its functionality, offering a more granular level of control over web requests. The evolution of these extensions laid the foundation foruBlock, combining elements from bothHTTP SwitchboardanduMatrixwhile streamlining the user experience for broader accessibility. Development ofuMatrixcontinued for several years alongside development ofuBlock Origin, withuMatrixbeing posed as a blocking option that wasdefinitely for advanced users. In a public discussion on 2 August, 2020, lead developer Raymond Hill stated that he lacked time to work on uMatrix due to beinga project large enough that [he] would be able to work on it only if [he] wasn't working on uBO.In response to a further comment, Hill stated he would never pass development to anyone after past experience, giving concerns such as monetisation and feature bloat, and stating he would insteadat mostarchive it, offering others the chance to fork the project.[20] Development onuMatrixended on July 21, 2021 without announcement, with the project's GitHub repository being converted to a public archive. uBlock[note 1]was developed by Raymond Hill to use community-maintainedblock lists,[21]while adding features and raising thecode qualitytoreleasestandards.[22]First released in June 2014 as a Chrome and Operaextension, in 2015 the extension became available in other browsers.[citation needed] A jointSourcepointandcomScoresurvey reported an 833% growth from November 2014 to August 2015, the strongest growth among adblockers listed.[23]The report attributed the growth to the desire of users for pure blockers, outside the "acceptable ads" program operated byAdblock Plus.[24] The development of uBlock stopped in August 2015 after its April break with uBlock Origin but there were updates again starting in January 2017.[25] In July 2018, ublock.org was acquired byAdBlockand resumed development.[26]From February 2019, uBlock began allowing users to participate in "acceptable ads",[27][28]a program run by Adblock Plus that allows some ads deemed "acceptable" and nonintrusive, and for which larger publishers pay a fee.[29] On April 3, 2015, Raymond Hill transferred the uBlock project to Chris Aljoudi due to frustrations with managing increasing user requests.[30]Hill explained that the projects were no longer a hobby but had become more like a tedious job. He stated,“These projects are to me, not a full-time job. It stopped being a hobby when it felt more and more like a tedious job. I will keep maintaining my version (and share with whoever cares to use it) because it guarantees the tool will match what I want out of it.”[31]On April 6, Hill created his own fork of the project, renaming it uBlock Origin.[32] Since October 2017, uBlock Origin has been completely separated from Aljoudi's uBlock.[33]Aljoudi created ublock.org to host and promote uBlock and to request donations. In response, uBlock's founder Raymond Hill stated that"the donations sought by ublock.org are not benefiting any of those who contributed most to create uBlock Origin."[6] In 2023, Google made changes known as "Manifest V3" to the WebRequest API used byad blockingand privacy extensions to block and modify network connections.[34][35]Following Google's implementation of Manifest V3 and the end of support for V2 (which was ultimately implemented in March 2025),[36]uBlock Origin's effectiveness is drastically reduced inGoogle Chromeand otherChromium-based browsers.[37][38][39] As a result, uBlock Origin Lite was created and designed to comply with the Manifest V3 (MV3) extension framework. uBO Lite differs significantly from uBO in several key aspects, primarily due to the constraints and design goals associated with MV3. Specifically, it lacks filter list updates outside of extension updates, and has no custom filters, strict-blocked pages, per-site switches, or dynamic filtering. Google has been criticized for implementing some of these features due to its domination in the online advertising market.[40][41][42][43] As of December 2024, the Chrome version of uBlock Origin Lite has over 4 million active users.[44][better source needed] uBlock Origin supports the majority ofAdblock Plus'sfilter syntax. The popular filter listsEasyListandEasyPrivacyare enabled by default. The extensions are capable of importinghosts filesand a number of community-maintained lists are available at installation. Among the host files available,Peter Lowe's ad servers & tracking listand lists of malware domains such as uBlock Origin's own anti-malicious filter calledBadware risksare also enabled as default, preventing users from visiting malicious websites such as those used for phishing, scams, malware, and more.[45]Some additional features include dynamic filtering of scripts andiframesand a tool for webpage element hiding.[citation needed] uBlock Origin offers several features not found in the original uBlock. These include: There used to be a feature that protected against IP address leaks viaWebRTC. However, this feature was removed in version 1.38 for all platforms exceptAndroid, as most browsers no longer have vulnerabilities related to WebRTC leaks.[46] Site-specific switches to toggle the blocking ofpop-ups, strict domain blocking, cosmetic filtering, blocking remote fonts, and JavaScript disabling were also added to uBlock Origin.[47]The Firefox version of uBlock Origin has an extra feature which helps to foil attempts by web sites to circumvent blockers.[48] uBlock Origin also supports modifying the response body of a request using the=replacedirective. This allows it to effectively alter and deliver the modified response directly to the browser. However, this functionality is currently supportedonlyon Firefox and Firefox-based browsers.[49] Technology websites and user reviews for uBlock Origin have regarded the extension as less resource-intensive than extensions that provide similar feature sets such as Adblock Plus.[50][51][52]Abenchmark test, conducted in August 2015 with ten blocking extensions, showed uBlock Origin as the most resource-efficient among the extensions tested.[53] uBlock Origin surveys what style resources are required for an individualweb pagerather than relying on a universalstyle sheet. The extension takes asnapshotof the filters the user has enabled, which contributes to accelerated browser start-up speed when compared to retrieving filters from cache every time.[54] Starting with version 1.54, uBlock Origin introduced the implementation ofdifferential updates (delta updates), which allows the extension to fetch only changes to filter lists rather than downloading the entire list. This results in lower bandwidth consumption and more frequent updates. This change was part of uBlock Origin's efforts to reduce both requests and bandwidth usage, with the goal of not being ranked among the most bandwidth-intensive projects onjsDelivr, as reflected in public statistics.[55][56] According to a research paper, uBlock Origin was able to reduce web page load times by 28.5%, the highest efficiency compared to other ad blockers likeAdBlock PlusandPrivacy Badger. Its use could significantly reduce energy consumption, saving an estimated 100 hours of browsing time annually for the average global user.[57] If widely adopted in theUnited States, it could save over $117 million per year in electricity costs, and globally, it could result in savings exceeding $1.8 billion. The research report also suggests that widespread use of uBlock Origin could have environmental benefits, potentially preventing pollution-related deaths caused by coal-fired power generation used to support the extra energy consumption from ads.[57] According to the creator and lead developer Raymond Hill of the extension, uBlock Origin works best onFirefox.[58]The main reason is that uBlock Origin (uBO) faces several technical limitations when used onChromium-based browserscompared to its performance on Firefox, and as a result, users may experience less effective ad blocking, potential exposure to unwanted content, and slower performance when using uBO on Chromium-based browsers:[59] In November 2019, a uBlock Origin user reported a novel technique used by some sites to bypass third-party tracker blocking. These sites link to URLs that are sub-domains of the page's domain, but those sub-domains resolve to third-party hosts via aCNAME record. Since the initial URL contained a sub-domain of the current page, it was interpreted by browsers as a first-party request and so was allowed by the filtering rules in uBlock Origin (and in similar extensions). The uBlock Origin developer came up with a solution using a DNS API which is exclusive to Firefox 60+.[60]The new feature was implemented in uBlock Origin 1.25, released on February 19, 2020.[61] Chromium-based browsers lack the ability to uncloak third-party servers disguised as first-party throughCNAME records. This limitation reduces the efficiency of blocking third-party trackers, a capability that uBO fully utilizes on Firefox. Furthermore: uBlock Origin is a widely praised content-filtering extension, known for being less memory-intensive than similar tools.[62][63][64][65]Its primary purpose is to give users control over their content-filtering choices, particularly in blocking advertisements and enhancing privacy.[66][67] The extension has received positive feedback from both users and academic researchers. It is frequently recognized for its effectiveness in blocking ads, improving privacy, and boosting browsing efficiency.[50][51][52]Numerous studies and scientific papers have examined uBlock Origin's impact on web performance, privacy, and user experience. These studies have contributed to a better understanding of the benefits of ad-blocking tools, with uBlock Origin often cited as one of the most comprehensive and efficient solutions. Its widespread use and inclusion in academic research highlight its importance in the broader context of digital privacy and online security.[68] uBlock Origin is also appreciated for its commitment to remaining a non-profit project.[69]It does not accept paid advertisements and refuses donations, encouraging users to support the maintainers of block lists directly.[70]This commitment to independence helps reinforce its reputation as a privacy-focused tool without commercial interests. Additionally, it has been praised for its positive environmental impact. By reducing web page load times, it can lead to significant energy savings. Widespread adoption of the tool could help lower energy consumption and mitigate environmental harm.[57] uBlock Origin is actively developed for applications based on two major layout engines.[21][50][71] With Google's phaseout of Manifest V2 in favor of Manifest V3, non-Chromium browsers likeFirefoxare unaffected.[72]As of April 2025,Microsoft Edgesupports uBlock Origin.[73]Braveis another Chromium browsers planning to continue supporting Manifest V2 for certain extensions, including uBlock Origin.[74] etc. etc.	https://en.wikipedia.org/wiki/HTTP_Switchboard
Google Chromeis aweb browserdeveloped byGoogle. It was first released in 2008 forMicrosoft Windows, built withfree softwarecomponents fromApple WebKitandMozilla Firefox.[15]Versions were later released forLinux,macOS,iOS,iPadOS, and also forAndroid, where it is the default browser.[16]The browser is also the main component ofChromeOS, where it serves as the platform forweb applications. Most of Chrome'ssource codecomes from Google'sfree and open-source softwareprojectChromium, but Chrome is licensed as proprietaryfreeware.[14]WebKitwas the originalrendering engine, but Google eventuallyforkedit to create theBlinkengine;[17]all Chrome variants except iOS used Blink as of 2017.[18] As of April 2024[update],StatCounterestimates that Chrome has a 65% worldwidebrowser market share(after peaking at 72.38% in November 2018) onpersonal computers(PC),[19]is most used on tablets (having surpassedSafari), and is also dominant on smartphones.[20][21]With a market share of 65% across all platforms combined, Chrome is the most used web browser in the world today.[22] Google chief executiveEric Schmidtwas previously involved in the "browser wars", a part of U.S.corporate history, and opposed the expansion of the company into such a new area. However, Google co-foundersSergey BrinandLarry Pagespearheaded a software demonstration that pushed Schmidt into making Chrome a core business priority, which resulted in commercial success.[23]Because of the proliferation of Chrome, Google has expanded the "Chrome" brand name to other products. These include not just ChromeOS but alsoChromecast,Chromebook,Chromebit,Chromebox, andChromebase. Google chief executive Eric Schmidt opposed the development of an independent web browser for six years. He stated that "at the time, Google was a small company", and he did not want to go through "bruising browser wars". Company co-founders Sergey Brin and Larry Page hired severalMozilla Firefoxdevelopers and built a demonstration of Chrome. Afterwards, Schmidt said, "It was so good that it essentially forced me to change my mind."[23] In September 2004, rumors of Google building a web browser first appeared. Online journals and U.S. newspapers stated at the time that Google was hiring former Microsoft web developers among others. It also came shortly after the release of Mozilla Firefox 1.0, which was surging in popularity and taking market share fromInternet Explorer, which had noted security problems.[24] Chrome is based on the open-source code of the Chromium project.[15]Development of the browser began in 2006,[25]spearheaded bySundar Pichai.[26]Chrome was "largely developed" in Google'sKitcheneroffice.[27] The release announcement was originally scheduled for September 3, 2008, and a comic byScott McCloudwas to be sent to journalists and bloggers explaining the features within the new browser.[28]Copies intended for Europe were shipped early andGermanblogger Philipp Lenssen of Google Blogoscoped made a scanned copy of the 38-page comic available on his website after receiving it on September 1, 2008.[29][30]Google subsequently made the comic available onGoogle Books,[31]and mentioned it on their official blog along with an explanation for the early release.[32]The product was named "Chrome" as an initial development projectcode name, because it is associated with fast cars and speed. Google kept the development project name as the final release name, as a "cheeky" or ironic moniker, as one of the main aims was to minimize theuser interface chrome.[33] The browser was first publicly released, officially as abeta version,[34]on September 2, 2008, forWindows XPand newer, and with support for 43 languages, and later as a "stable" public release on December 11, 2008. On that same day, aCNETnews item drew attention to a passage in the Terms of Service statement for the initial beta release, which seemed to grant to Google a license to all content transferred via the Chrome browser.[35]This passage was inherited from the general Google terms of service.[36]Google responded to this criticism immediately by stating that the language used was borrowed from other products, and removed this passage from the Terms of Service.[14] Chrome quickly gained about 1% usage share.[32][37][38]After the initial surge, usage share dropped until it hit a low of 0.69% in October 2008. It then started rising again and by December 2008, Chrome again passed the 1% threshold.[39]In early January 2009, CNET reported that Google planned to release versions of Chrome for macOS and Linux in the first half of the year.[40]The first official macOS and Linux developer previews of Chrome were announced on June 4, 2009,[41]with a blog post saying they were missing many features and were intended for early feedback rather than general use.[42]In December 2009, Google released beta versions of Chrome for macOS and Linux.[43][44]Google Chrome 5.0, announced on May 25, 2010, was the first stable release to support all three platforms.[45] Chrome was one of the twelve browsers offered onBrowserChoice.eutoEuropean Economic Areausers of Microsoft Windows in 2010.[46] Chrome was assembled from 25 different code libraries from Google and third parties such asMozilla'sNetscape Portable Runtime,Network Security Services,NPAPI(dropped as of version 45),[47]Skia Graphics Engine,SQLite, and a number of other open-source projects.[48]TheV8JavaScriptvirtual machinewas considered a sufficiently important project to be split off (as wasAdobe/Mozilla'sTamarin) and handled by a separate team in Denmark coordinated byLars Bak. According to Google, existing implementations were designed "for small programs, where the performance and interactivity of the system weren't that important", but web applications such asGmail"are using the web browser to the fullest when it comes toDOMmanipulations and JavaScript", and therefore would significantly benefit from a JavaScript engine that could work faster. Chrome initially used the WebKit rendering engine to display web pages. In 2013, they forked the WebCore component to create their own layout engine Blink. Based on WebKit, Blink only uses WebKit's "WebCore" components, while substituting other components, such as its own multi-process architecture, in place of WebKit's native implementation.[17]Chrome is internally tested withunit testing, automated testing of scripted user actions,fuzz testing, as well as WebKit's layout tests (99% of which Chrome is claimed to have passed), and against commonly accessed websites inside the Google index within 20–30 minutes.[31]Google createdGearsfor Chrome, which added features forweb developerstypically relating to the building of web applications, including offline support.[31]Google phased out Gears as the same functionality became available in theHTML5standards.[49] In March 2011, Google introduced a new simplified logo to replace the previous 3D logo that had been used since the project's inception. Google designer Steve Rura explained the company reasoning for the change: "Since Chrome is all about making your web experience as easy and clutter-free as possible, we refreshed the Chrome icon to better represent these sentiments. A simpler icon embodies the Chrome spirit – to make the web quicker, lighter, and easier for all."[50] On January 11, 2011, the Chrome product manager, Mike Jazayeri, announced that Chrome would removeH.264 video codecsupport for its HTML5 player, citing the desire to bring Google Chrome more in line with the currently available open codecs available in the Chromium project, which Chrome is based on.[51]Despite this, on November 6, 2012, Google released a version of Chrome on Windows which addedhardware-acceleratedH.264 video decoding.[52]In October 2013,Ciscoannounced that it was open-sourcing its H.264 codecs, and it would cover all fees required.[53] On February 7, 2012, Google launchedGoogle Chrome BetaforAndroid 4.0devices.[54]On many new devices withAndroid 4.1or later preinstalled, Chrome is the default browser.[55]In May 2017, Google announced a version of Chrome foraugmented realityandvirtual realitydevices.[56] Google Chrome features a minimalistic user interface, with its user-interface principles later being implemented into other browsers. For example, the merging of theaddress barand search bar into theomniboxoromnibar[57][58]Chrome also has a reputation for strong browser performance.[59][60] The first release of Google Chrome passed both theAcid1andAcid2tests. Beginning with version 4.0, Chrome passed all aspects of theAcid3test,[61]However as of April 2017 Chrome no longer passes Acid3 due to changing consensus on Web standards.[62][63] As of May 2011[update], Chrome has very good support for JavaScript/ECMAScriptaccording toEcma International's ECMAScript standards conformance Test 262[64](version ES5.1 May 18, 2012). This test reports as the final score the number of tests a browser failed; hence lower scores are better. In this test, Chrome version 37 scored 10 failed/11,578 passed. For comparison, Firefox 19 scored 193 failed/11,752 passed and Internet Explorer 9 had a score of 600+ failed, while Internet Explorer 10 had a score of 7 failed. In 2011, on the official CSS 2.1 test suite by standardization organizationW3C, WebKit, the Chrome rendering engine, passed 89.75% (89.38% out of 99.59% covered) CSS 2.1 tests.[65] On the HTML5 web standards test, Chrome 41 scored 518 out of 555 points, placing it ahead of the five most popular desktop browsers.[66][67]Chrome 41 on Android scored 510 out of 555 points.[68][69][70]Chrome 44 scored 526, only 29 points less than the maximum score.[71] By default, the mainuser interfaceincludes back, forward, refresh/cancel and menu buttons. A home button is not shown by default, but can be added through the Settings page to take the user to the new tab page or a custom home page.[72] Tabsare the main component of Chrome's user interface and have been moved to the top of the window rather than below the controls. This subtle change contrasts with many existing tabbed browsers which are based onwindowsand contain tabs. Tabs, with their state, can be transferred between window containers by dragging. Each tab has its own set of controls, including theOmnibox.[31] TheOmniboxis a URL box that combines the functions of both the address bar and search box. If a user enters the URL of a site previously searched from, Chrome allows pressingTabto search the site again directly from the Omnibox. When a user starts typing in the Omnibox, Chrome provides suggestions for previously visited sites (based on the URL or in-page text), popular websites (not necessarily visited before – powered byGoogle Instant), and popular searches. Although Instant can be turned off, suggestions based on previously visited sites cannot be turned off. Chrome will alsoautocompletethe URLs of sites visited often.[31]If a user types keywords into the Omnibox that do not match any previously visited websites and presses enter, Chrome will conduct the search using the default search engine. One of Chrome's differentiating features is theNew Tab Page, which can replace the browserhome pageand is displayed when a new tab is created. Originally, this showed thumbnails of the nine most visited websites, along with frequent searches, recent bookmarks, and recently closed tabs; similar to Internet Explorer and Firefox withGoogle Toolbar, orOpera'sSpeed Dial.[31]In Google Chrome 2.0, the New Tab Page was updated to allow users to hide thumbnails they did not want to appear.[73] Starting in version 3.0, the New Tab Page was revamped to display thumbnails of the eight most visited websites. The thumbnails could be rearranged, pinned, and removed. Alternatively, a list of text links could be displayed instead of thumbnails. It also features a "Recently closed" bar that shows recently closed tabs and a "tips" section that displays hints and tricks for using the browser.[74]Starting with Google Chrome 3.0, users can install themes to alter the appearance of the browser.[75]Many free third-party themes are provided in an online gallery,[76]accessible through a "Get themes" button in Chrome's options.[77] Chrome includes abookmarkssubmenu that lists the user's bookmarks, provides easy access to Chrome'sBookmark Manager,and allows the user to toggle abookmarks baron or off. On January 2, 2019, Google introduced Native Dark Theme for Chrome onWindows 10.[78] In 2023, it was announced that Chrome would be completely revamped, using Google'sMaterial Youdesign language, the revamp would include more rounded corners, Chrome colors being swapped out for a similar dynamic color system introduced inAndroid 12, a revamped address bar, new icons and tabs, and a more simplified 3 dot menu.[79] Starting with Google Chrome 4.1, the application added a built-in translation bar usingGoogle Translate. Language translation is currently available for 52 languages.[80]When Chrome detects a foreign language other than the user's preferred language set during the installation time, it asks the user whether or not to translate. Chrome allows users to synchronize their bookmarks, history, and settings across all devices with the browser installed by sending and receiving data through a chosen Google Account, which in turn updates all signed-in instances of Chrome. This can be authenticated either through Google credentials, or a sync passphrase. For web developers, Chrome has an element inspector which allows users to look into the DOM and see what makes up the webpage.[81] Chrome has special URLs that load application-specific pages instead of websites or files on disk. Chrome also has a built-in ability to enable experimental features. Originally calledabout:labs, the address was changed toabout:flagsto make it less obvious to casual users.[82][83] The desktop edition of Chrome is able to save pages as HTML with assets in a "_files" subfolder, or as unprocessed HTML-only document. It also offers an option to save in theMHTMLformat.[84] Chrome allows users to make local desktopshortcutsthat open web applications in the browser. The browser, when opened in this way, contains none of the regular interface except for the title bar, so as not to "interrupt anything the user is trying to do". This allows web applications to run alongside local software (similar toMozilla PrismandFluid).[31] This feature, according to Google, would be enhanced with theChrome Web Store, a one-stop web-based web applications directory which opened in December 2010.[85][86] In September 2013, Google started making Chrome apps "For your desktop". This meant offline access, desktop shortcuts, and less dependence on Chrome—apps launch in a window separate from Chrome, and look more like native applications.[87] Announced on December 7, 2010, the Chrome Web Store allows users to install web applications as extensions to the browser, although most of these extensions function simply as links to popular web pages or games, some of the apps likeSpringpaddo provide extra features like offline access. The themes and extensions have also been tightly integrated into the new store, allowing users to search the entire catalog of Chrome extras.[88] The Chrome Web Store was opened on February 11, 2011, with the release of Google Chrome 9.0.[89] Browser extensionsare able to modify Google Chrome. They are supported by the browser's desktop edition,[90]but not on mobile. These extensions are written using web technologies likeHTML, JavaScript, andCSS.[91]They are distributed through Chrome Web Store,[92]initially known as the Google Chrome Extensions Gallery.[90]Some extensions focus on providing accessibility features. Google Tone is an extension developed by Google that when enabled, can use a computer's speakers to exchangeURLswith nearby computers with an Internet connection that have the extension enabled as well.[93][94] On September 9, 2009, Google enabled extensions by default on Chrome's developer channel, and provided several sample extensions for testing.[95]In December, the Google Chrome Extensions Gallery beta began with approximately 300 extensions.[44][96]It was launched on January 25, 2010, along with Google Chrome 4.0, containing approximately 1500 extensions.[97] In 2014, Google started preventing some Windows users from installing extensions not hosted on the Chrome Web Store.[98][99]The following year Google reported a "75% drop in customer support help requests for uninstalling unwanted extensions" which led them to expand this restriction to all Windows and Mac users.[100] In October 2018, Google announced a major future update to Chrome's extensionAPI, known as "Manifest V3" (in reference to themanifest filecontained within extensions). Manifest V3 is intended to modernize the extension architecture and improve the security and performance of the browser; it adopts declarative APIs to "decrease the need for overly-broad access and enable more performant implementation by the browser", replaces background pages with feature-limited "Service Workers" to reduce resource usage, and prohibits remotely-hosted code.[101][102][103] Google faced a criticism for this change since it limits the number of rules and types of expressions that may be checked by adblockers. Additionally, the prohibition of remotely-hosted code will restrict the ability for adblocking filter lists to be updated independently of the extension itself.[104][105] The JavaScript virtual machine used by Chrome, the V8 JavaScript engine, has features such asdynamic code generation,hidden class transitions, andprecise garbage collection.[31] In 2008, several websites performed benchmark tests using theSunSpider JavaScript Benchmarktool as well as Google's own set of computationally intense benchmarks, which includeray tracingandconstraint solving.[109]They unanimously reported that Chrome performed much faster than all competitors against which it had been tested, includingSafari(for Windows),Firefox 3.0,Internet Explorer 7, Opera, andInternet Explorer 8.[110][111][112][59][113][114]However, on October 11, 2010, independent tests of JavaScript performance, Chrome has been scoring just behind Opera'sPrestoengine since it was updated in version 10.5.[115] On September 3, 2008, Mozilla responded by stating that their ownTraceMonkeyJavaScript engine (then in beta), was faster than Chrome's V8 engine in some tests.[116][117][118]John Resig, Mozilla's JavaScript evangelist, further commented on the performance of different browsers on Google's own suite, commenting on Chrome's "decimating" of the other browsers, but he questioned whether Google's suite was representative of real programs. He stated that Firefox 3.0 performed poorly onrecursion-intensive benchmarks, such as those of Google, because the Mozilla team had not implemented recursion-tracing yet.[119] Two weeks after Chrome's launch in 2008, the WebKit team announced a new JavaScript engine,SquirrelFish Extreme,[120]citing a 36% speed improvement over Chrome's V8 engine.[121][122][123] Like most major web browsers, Chrome usesDNSprefetching to speed up website lookups,[81]as do other browsers like Firefox,[124]Safari,[125]Internet Explorer (called DNS Pre-resolution),[126]and in Opera as a UserScript (not built-in).[127] Chrome formerly used their now-deprecatedSPDYprotocol instead of onlyHTTP[128][129]when communicating with servers that support it, such as Google services, Facebook, Twitter. SPDY support was removed in Chrome version 51. This was due to SPDY being replaced byHTTP/2, a standard that was based upon it. In November 2019, Google said it was working on several "speed badging" systems that let visitors know why a page is taking time to show up. The variations include simple text warnings and more subtle signs that indicate a site is slow. No date has been given for when the badging system will be included with the Chrome browser.[130] Chrome formerly supported a Data Saver feature for making pages load faster called Lite Mode.[131]Previously, Chrome engineers Addy Osmani and Scott Little announced Lite Mode would automatically lazy-load images and iframes for faster page loads.[132]Lite Mode was switched off in Chrome 100, citing a decrease in mobile data costs for many countries.[133] Chrome periodically retrieves updates of twoblacklists(one forphishingand one formalware), and warns users when they attempt to visit a site flagged as potentially harmful. This service is also made available for use by others via a free public API called "Google Safe BrowsingAPI".[31] Chrome uses a process-allocation model tosandboxtabs.[134]Using theprinciple of least privilege, each tab process cannot interact with critical memory functions (e.g. OS memory, user files) or other tab processes – similar to Microsoft's "Protected Mode" used byInternet Explorer 9or greater. TheSandbox Teamis said to have "taken this existing process boundary and made it into ajail". This enforces acomputer security modelwhereby there are two levels ofmultilevel security(userandsandbox) and thesandboxcan only respond to communication requests initiated by theuser.[135]On Linux sandboxing uses theseccompmode.[136][137] In January 2015,TorrentFreakreported that using Chrome when connected to the internet using a VPN can be a serious security issue due to the browser's support forWebRTC.[138] On September 9, 2016, it was reported that starting with Chrome 56, users will be warned when they visit insecure HTTP websites to encourage more sites to make the transition to HTTPS.[139] On December 4, 2018, Google announced its Chrome 71 release with new security features, including a built-in ad featuring system. In addition, Google also announced its plan to crack down on websites that make people involuntarily subscribe to mobile subscription plans.[140] On September 2, 2020, with the release of Chrome 85, Google extended support for Secure DNS in Chrome for Android. DNS-over-HTTPS (DoH), was designed to improve safety and privacy while browsing the web. Under the update, Chrome automatically switches to DNS-over-HTTPS (DoH), if the current DNS provider supports the feature.[141] Since 2008, Chrome has been faulted for not including a master password to prevent casual access to a user's passwords. Chrome developers have indicated that a master password does not provide real security against determined hackers and have refused to implement one. Bugs filed on this issue have been marked "WontFix".[142][143]As of February 2014[update], Google Chrome asks the user to enter their Windows account password before showing saved passwords.[144] On Linux, Google Chrome/Chromium can store passwords in three ways:GNOME Keyring,KWalletorplain text. Google Chrome/Chromium chooses which store to use automatically, based on thedesktop environmentin use.[145]Passwords stored in GNOME Keyring or KWallet are encrypted on disk, and access to them is controlled by dedicated daemon software. Passwords stored in plain text are not encrypted. Because of this, when either GNOME Keyring or KWallet is in use, any unencrypted passwords that have been stored previously are automatically moved into the encrypted store. Support for using GNOME Keyring and KWallet was added in version 6, but using these (when available) was not made the default mode until version 12. As of version 45, the Google Chrome password manager is no longer integrated withKeychain, since theinteroperabilitygoal is no longer possible.[146] No security vulnerabilities in Chrome were exploited in the three years ofPwn2Ownfrom 2009 to 2011.[147]At Pwn2Own 2012, Chrome was defeated by aFrenchteam who usedzero day exploitsin the version of Flash shipped with Chrome to take complete control of a fully patched64-bitWindows 7 PC using a booby-trapped website that overcame Chrome's sandboxing.[148] Chrome was compromised twice at the 2012 CanSecWestPwnium.[148][149][150]Google's official response to the exploits was delivered by Jason Kersey, who congratulated the researchers, noting "We also believe that both submissions are works of art and deserve wider sharing and recognition."[151]Fixes for these vulnerabilities were deployed within 10 hours of the submission.[152][153] A significant number of security vulnerabilities in Chrome occurred in theAdobe Flash Player. For example, the 2016 Pwn2Own successful attack on Chrome relied on four security vulnerabilities. Two of the vulnerabilities were in Flash, one was in Chrome, and one was in the Windows kernel.[154]In 2016, Google announced that it was planning to phase out Flash Player in Chrome, starting in version 53. The first phase of the plan was to disable Flash for ads and "background analytics", with the ultimate goal of disabling it completely by the end of the year, except on specific sites that Google has deemed to be broken without it. Flash would then be re-enabled with the exclusion of ads and background analytics on a site-by-site basis.[155] Leaked documents from 2013 to 2016 codenamedVault 7detail the capabilities of the United StatesCentral Intelligence Agency, such as the ability to compromise web browsers (including Google Chrome).[156][157] Google introduced download scanning protection in Chrome 17.[158]In February 2018, Google introduced an ad blocking feature based on recommendations from theInteractive Advertising Bureau. Sites that employ invasive ads are given a 30-day warning, after which their ads will be blocked.[159]Consumer Reportsrecommended users install dedicated ad-blocking tools instead, which offer increased security against malware and tracking.[160] Theprivate browsingfeature calledIncognitomode prevents the browser from locally storing anyhistoryinformation,cookies, site data, or form inputs.[172]Downloaded files and bookmarks will be stored. In addition, user activity is not hidden from visited websites or the Internet service provider.[173] Incognito mode is similar to the private browsing feature in other web browsers. It does not prevent saving in all windows: "You can switch between an incognito window and any regular windows you have open. You'll only be in incognito mode when you're using the incognito window".[174] The iOS version of Chrome also supports the optional ability to lock incognito tabs with Face ID, Touch ID or the device's passcode.[175] In February 2012, Google announced that Chrome would implement theDo Not Track(DNT) standard to inform websites the user's desire not to be tracked. The protocol was implemented in version 23. In line with the W3's draft standard for DNT,[176]it is turned off by default in Chrome.[177] A multi-process architecture is implemented in Chrome where, by default, a separate process is allocated to each site instance and plugin.[178]This procedure is termedprocess isolation,[179]and raises security and stability by preventing tasks from interfering with each other. An attacker successfully gaining access to one application gains access to no others,[180]and failure in one instance results in aSad Tabscreen of death, similar to the well-knownSad Mac, but only one tab crashes instead of the whole application. This strategy exacts a fixed per-process cost up front, but results in less memory bloat over time as fragmentation is confined to each instance and no longer needs further memory allocations.[31]This architecture was later adopted in Safari[181]and Firefox.[182] Chrome includes aprocess managementutility calledTask Managerwhich lets users see what sites and plugins are using the mostmemory,downloadingthe mostbytesand overusing theCPUand provides the ability to terminate them.[183]Chrome Version 23 ensures its users an improved battery life for the systems supporting Chrome's GPU accelerated video decoding.[184][52] The first production release on December 11, 2008, marked the end of the initial Beta test period and the beginning of production. Shortly thereafter, on January 8, 2009, Google announced an updated release system with three channels: Stable (corresponding to the traditional production), Beta, and Developer preview (also called the "Dev" channel). Where there were before only two channels: Beta and Developer, now there were three. Concurrently, all Developer channel users were moved to the Beta channel along with the promoted Developer release. Google explained that now the Developer channel builds would be less stable and polished than those from the initial Google Chrome's Beta period. Beta users could opt back to the Developer channel as desired. Each channel has its own release cycle and stability level. The Stable channel updated roughly quarterly, with features and fixes that passed "thorough" testing in the Beta channel. Beta updated roughly monthly, with "stable and complete" features migrated from the Developer channel. The Developer channel updated once or twice per week and was where ideas and features were first publicly exposed "(and sometimes fail) and can be very unstable at times". [Quoted remarks from Google's policy announcements.][185][186][187] On July 22, 2010, Google announced it would ramp up the speed at which it releases new stable versions; the release cycles were shortened from quarterly to six weeks for major Stable updates.[188]Beta channel releases now come roughly at the same rate as Stable releases, though approximately one month in advance, while Dev channel releases appear roughly once or twice weekly, allowing time for basic release-critical testing.[189]This faster release cycle also brought a fourth channel: the "Canary" channel, updated daily from a build produced at 09:00 UTC from the most stable of the last 40revisions.[190]The name refers to the practice ofusing canaries in coal mines, so if a change "kills" Chrome Canary, it will be blocked from migrating down to the Developer channel, at least until fixed in a subsequent Canary build.[191]Canary is "the most bleeding-edge official version of Chrome and somewhat of a mix between Chrome dev and the Chromium snapshot builds". Canary releases run side by side with any other channel; it is not linked to the other Google Chrome installation and can therefore run different synchronization profiles, themes, and browser preferences. This ensures that fallback functionality remains even when some Canary updates may contain release-breaking bugs.[192]It does not natively include the option to be the default browser, although on Windows and macOS it can be set through System Preferences. Canary was Windows-only at first; a macOS version was released on May 3, 2011.[193] The Chrome beta channel for Android was launched on January 10, 2013; like Canary, it runs side by side with the stable channel for Android.[194][195]Chrome Dev for Android was launched on April 29, 2015.[196] All Chrome channels are automatically distributed according to their respective release cycles. The mechanism differs by platform. On Windows, it uses Google Update, and auto-update can be controlled viaGroup Policy.[197]Alternatively, users may download a standalone installer of a version of Chrome that does not auto-update.[198][199]On macOS, it uses Google Update Service, and auto-update can be controlled via the macOS "defaults" system.[200]On Linux, it lets the system's normalpackage management systemsupply the updates. This auto-updating behavior is a key difference from Chromium, the non-brandedopen-sourcebrowser which forms the core of Google Chrome. Because Chromium also serves as thepre-releasedevelopment trunkfor Chrome, its revisions are provided as source code and buildable snapshots are produced continuously with each newcommit, requiring users to manage their own browser updates.[201] In March 2021, Google announced that starting with Chrome 94 in the third quarter of 2021, Google Chrome Stable releases will be made every four weeks, instead of six weeks as they have been since 2010. Also, Google announced a new release channel for system administrators and browser embedders with releases every eight weeks.[202] Releases are identified by a four-part version number, e.g. 42.0.2311.90 (Windows Stable release April 14, 2015[203]). The components are major.minor.build.patch.[204][205] Chromium and Chrome release schedules are linked through Chromium (Major) versionBranch Pointdates, published annually.[204]The Branch Points precede the final Chrome Developer build (initial) release by 4 days (nearly always) and the Chrome Stable initial release by roughly 53 days.[206] Example: The version 42 Branch Point was February 20, 2015.[204]Developer builds stopped advancing atbuild2311 with release 42.0.2311.4 on February 24,[207]4 days later. The first Stable release, 42.0.2311.90, was April 14, 2015,[203]53 days after the Branch Point. Chrome supportscolor managementby using the system-provided ICC v2 and v4 support on macOS, and from version 22 supports ICC v2 profiles by default on other platforms.[208] In Chrome, when not connected to the Internet and an error message displaying "No internet" is shown, on the top, an "8-bit"Tyrannosaurusrexis shown, but when pressing the space bar on a keyboard, mouse-clicking on it or tapping it on touch devices, the T-Rex instantly jumps once and dashes across acactus-ridden desert, revealing it to be anEaster eggin the form of aplatform game.[209][210][211][212]The game itself is an infinite runner, and there is no time limit in the game as it progresses faster and periodically tints to a black background. A school or enterprise manager can disable the game.[213] The current version of Chrome runs on: As of April 2016[update], stable 32-bit and 64-bit builds are available for Windows, with only 64-bit stable builds available for Linux and macOS.[215][216][217]64-bit Windows builds became available in the developer channel and as canary builds on June 3, 2014,[218]in the beta channel on July 30, 2014,[219]and in the stable channel on August 26, 2014.[220]64-bit macOS builds became available as canary builds on November 7, 2013,[221]in the beta channel on October 9, 2014,[222]and in the stable channel on November 18, 2014.[215] Starting with the release of version 89, Chrome will only be supported onIntel/Intel x86andAMDprocessors with theSSE3instruction set.[223][224][225] A beta version for Android 4.0 devices was launched on February 7, 2012, available for a limited number of countries fromGoogle Play.[237][238] Notable features: synchronization with desktop Chrome to provide the same bookmarks and view the same browser tabs,[239]page pre-rendering,[240]hardware acceleration.[241] Many of the latest HTML5 features: almost all of the Web Platform's features: GPU-accelerated canvas, including CSS 3D Transforms, CSS animations, SVG,WebSocket(including binary messages), Dedicated Workers; it has overflow scroll support, strong HTML5 video support, and new capabilities such as IndexedDB, WebWorkers, Application Cache and the File APIs, date- and time-pickers, parts of the Media Capture API.[240][242]Also supports mobile oriented features such as Device Orientation and Geolocation.[242] Mobile customizations: swipe gesture tab switching,[239]link preview allows zooming in on (multiple) links to ensure the desired one is clicked,[239]font size boosting to ensure readability regardless of the zoom level.[242] Features missing in the mobile version include sandboxed tabs,[240]Safe Browsing,[240]apps or extensions,[241]Adobe Flash (now and in the future),[241]Native Client,[241]and the ability toexportuser data such a list of their opened tabs or their browsing history intoportablelocal files.[243] Development changes: remote debugging,[240][244]part of the browser layer has been implemented in Java, communicating with the rest of the Chromium and WebKit code through Java Native Bindings.[242]The code of Chrome for Android is a fork of the Chromium project. It is a priority to upstream most new and modified code to Chromium and WebKit to resolve the fork.[242] The April 17, 2012, update included availability in 31 additional languages and in all countries where Google Play is available. A desktop version of a website can also be requested as opposed to a mobile version. In addition, Android users can now add bookmarks to their Android home screens if they choose and decide which apps should handle links opened in Chrome.[245] On June 27, 2012, Google Chrome for Android exited beta and became stable.[246][247] Chrome 18.0.1026311, released on September 26, 2012, was the first version of Chrome for Android to support mobile devices based on Intel x86.[248] Starting from version 25, the Chrome version for Android is aligned with the desktop version, and usually new stable releases are available at the same time between the Android and the desktop version. Google released a separate Chrome for Android beta channel on January 10, 2013, with version 25.[194]As of 2013[update]a separate beta version of Chrome is available in the Google Play Store – it can run side by side with the stable release.[249] Chrome is available onApple's mobile iOS and iPadOS operating systems. Released in theApple App Storeon June 26, 2012, it supports theiPad,iPhone, andiPod Touch, and the current version requires that the device has iOS 16.0 or greater installed.[250]In accordance with Apple's requirements for browsers released through their App Store, this version of Chrome uses theiOS WebKit– which is Apple's own mobile rendering engine and components, developed for theirSafaribrowser – therefore it is restricted from using Google's own V8 JavaScript engine.[251][252]Chrome is the default web browser for the iOS and iPadOSGmailapplication. In a review byChitika, Chrome was noted as having 1.5% of the iOS web browser market as of July 18, 2012[update].[253]In October 2013, Chrome had 3% of the iOS browser market.[254][needs update] OnLinux distributions, support for 32-bit Intel processors ended in March 2016 although Chromium is still supported.[255]As of Chrome version 26, Linux installations of the browser may be updated only on systems that supportGCCv4.6 andGTKv2.24 or later. Thus deprecated systems include (for example) Debian 6's 2.20, andRHEL6's 2.18.[256] Support for Google Chrome on Windows XP andWindows Vistaended in April 2016.[257]The last release of Google Chrome that can be run on Windows XP and Vista was version 49.0.2623.112,[258]released on April 7, 2016,[259]then re-released on April 11, 2016.[260] Support for Google Chrome onWindows 7was originally supposed to end upon on July 15, 2021.[261]However, the date was moved back to January 15, 2022, due to the ongoing COVID-19 pandemic. Since enterprises took more time to migrate toWindows 10or11, the end of support date was pushed back again until January 15, 2023.[262]Support for not only Windows 7, but also Windows 8 and 8.1 ended on this date. The last version to support these versions of Windows is Chrome 109.[263][264][265] "Windows 8 mode" was introduced in 2012 and has since been discontinued. It was provided to the developer channel, which enabledWindows 8and8.1users to run Chrome with a full-screen, tablet-optimized interface, with access to snapping, sharing, and search functionalities.[266]In October 2013, Windows 8 mode on the developer channel changed to use a desktop environment mimicking the interface of ChromeOS with a dedicated windowing system and taskbar for web apps.[267]This was removed on version 49 and users that have upgraded to Windows 10 will lose this feature.[268] Google dropped support forMac OS X 10.5with the release of Chrome 22.[269]Support for32-bitversions of Chrome ended in November 2014 with the release of Chrome 39.[270][271][215]Support forMac OS X 10.6,OS X 10.7, andOS X 10.8ended in April 2016 with the release of Chrome 50. Support forOS X 10.9ended in April 2018 with the release of Chrome 66. Support forOS X 10.10ended in January 2021 with the release of Chrome 88. Support forOS X 10.11andmacOS 10.12ended in August 2022 with the release of Chrome 104.[citation needed]Support formacOS 10.13andmacOS 10.14ended in September 2023 with the release of Chrome 117.[272]Support formacOS 10.15ended in September 2024 with the release of Chrome 129.[273] Google Chrome is the basis of Google's ChromeOS operating system that ships on specific hardware from Google's manufacturing partners.[274]The user interface has aminimalist designresembling the Google Chrome browser. ChromeOS is aimed at users who spend most of their computer time on the Web; the only applications on the devices are a browser incorporating a media player and afile manager.[275][276][277][278][279] Google announced ChromeOS on July 7, 2009.[280] Google Chrome was met with acclaim upon release. In 2008, Matthew Moore ofThe Daily Telegraphsummarized the verdict of early reviewers: "Google Chrome is attractive, fast and has some impressive new features..."[281] Initially, Microsoft reportedly played down the threat from Chrome and predicted that most people would embrace Internet Explorer 8.Opera Softwaresaid that "Chrome will strengthen the Web as the biggest application platform in the world".[282]But by February 25, 2010,BusinessWeekhad reported that "For the first time in years, energy and resources are being poured into browsers, the ubiquitous programs for accessing content on the Web. Credit for this trend – a boon to consumers – goes to two parties. The first is Google, whose big plans for the Chrome browser have shaken Microsoft out of its competitive torpor and forced the software giant to pay fresh attention to its own browser, Internet Explorer. Microsoft all but ceased efforts to enhance IE after it triumphed in the last browser war, sending Netscape to its doom. Now it's back in gear."[283]Mozilla said that Chrome's introduction into the web browser market comes as "no real surprise", that "Chrome is not aimed at competing with Firefox", and furthermore that it would not affectGoogle's revenue relationship with Mozilla.[284][285] Chrome's design bridges the gap between desktop and so-called "cloud computing." At the touch of a button, Chrome lets you make a desktop, Start menu, or QuickLaunch shortcut to any Web page or Web application, blurring the line between what's online and what's inside your PC. For example, I created a desktop shortcut for Google Maps. When you create a shortcut for a Web application, Chrome strips away all of the toolbars and tabs from the window, leaving you with something that feels much more like a desktop application than like a Web application or page. With its dominance in the web browser market, Google has been accused of using Chrome and Blink development to push new web standards that are proposed in-house by Google and subsequently implemented by its services first and foremost. These have led to performance disadvantages and compatibility issues with competing browsers, and in some cases, developers intentionally refusing to test their websites on any other browser than Chrome.[287]Tom Warren ofThe Vergewent as far as comparing Chrome toInternet Explorer 6, the default browser of Windows XP that was often targeted by competitors due to its similar ubiquity in the early 2000s.[288]In 2021, computer scientist and lawyerJonathan Mayerstated that Chrome has increasingly become an agent forGoogle LLCthan auser agent, as it is "the only major web browser that lacks meaningful privacy protections by default, shoves users toward linking activity with a Google Account, and implements invasive new advertising capabilities."[289] Aclass-actionlawsuit seeking $5 billion in damages was filed against Google in 2020 on the grounds it misled consumers into thinking it would not track them when using incognito mode, despite using various means to do so. In December 2023, a settlement was reportedly agreed to, with public disclosure expected in February 2024.[290][needs update] In June 2015, theDebiandeveloper community discovered that Chromium 43 and Chrome 43 were programmed to download theHotword Shared Module, which could enable theOK Googlevoice recognition extension, although by default it was "off". This raised privacy concerns in the media.[291][292]The module was removed in Chrome 45, which was released on September 1, 2015, and was only present in Chrome 43 and 44.[293][294] Chrome sends details about its users and their activities to Google through both optional and non-optional user tracking mechanisms.[295][296] Some of the tracking mechanisms can be optionally enabled and disabled through the installation interface[297]and through the browser's options dialog.[298]Unofficial builds, such asSRWare Iron, seek to remove these features from the browser altogether.[299]The RLZ library, which is used to measure the success of marketing promotions, is not included in the Chromium browser either.[300] In March 2010, Google devised a new method to collect installation statistics: the unique ID token included with Chrome is now used for only the first connection that Google Update makes to its server.[301] The optional suggestion service included in Google Chrome has been criticized because it provides the information typed into the Omnibox to the search provider before the user even hits return. This allows the search engine to provide URL suggestions, but also provides them with web use information tied to anIP address.[302] Chrome previously was able to suggest similar pages when a page could not be found. For this, in some cases Google servers were contacted.[303]The feature has since been removed.[citation needed] A 2019 review byWashington Posttechnology columnist Geoffrey A. Fowler found that in a typical week of browsing, Chrome allowed thousands more cookies to be stored than Mozilla Firefox. Fowler pointed out that because of its advertising businesses, despite the privacy controls it offers users, Google is a major producer of third-party cookies and has a financial interest in collecting user data; he recommended switching to Firefox, Apple Safari, or Chromium-basedBrave.[304] On installation Can be disabled in ChromeOS.[305]For Chrome browsers running in all other operating systems:[305] In 2023, Google proposed a technology that claims to "hide the IP and traffic of its users" by routing Chrome traffic to Google servers. This has drawn criticism as all traffic is readily available for Google to use.[309][310] Also tied with Google is its advertising business, which, given the vast market share of Chrome, sought to introduce features that protects this revenue stream, mainly the introduction of a cookie-tracking alternative named Federated Learning of Cohorts (FloC), which evolved into Topics, and Manifest V3 API changes for extensions. In January 2021, Google stated it was making progress on developing privacy-friendly alternatives which would replace third-party cookies currently being used by advertisers and companies to track browsing habits. Google then promised to phase out the use of cookies in their web-browser in 2022, implementing their FLoC technology instead. The announcement triggeredantitrustconcerns from multiple countries for abusing the Chrome browser's marketmonopoly, with theU.K.'s Competition and Markets Authorityand theEuropean Commissionboth opening formal probes.[311][312][313][314]The FLoC proposal also drew criticism fromDuckDuckGo, Brave, and the Electronic Frontier Foundation for underestimating the ability of the API to track users online.[315][316][317][318] On January 25, 2022, Google announced it had killed off development of its FLoC technologies and proposed the new Topics API to replace it. Topics is similarly intended to replace cookies, using one's weekly web activity to determine a set of five interests. Topics are supposed to refresh every three weeks, changing the type of ads served to the user and not retaining the gathered data.[319][320] Manifest V3 has faced criticism for changes to the WebRequest API used byad blockingand privacy extensions to block and modify network connections.[321]The declarative version of WebRequest uses rules processed by the browser, rather than sending all network traffic through the extension, which Google stated would improve performance. However, DeclarativeWebRequest is limited in the number of rules that may be set, and the types ofexpressionsthat may be used.[321]Additionally, the prohibition of remotely-hosted code will restrict the ability for filter lists to be updated independently of the extension itself. As the Chrome Web Store review process has an invariable length, filter lists may not be updated in a timely fashion.[322][323] Google has been accused of using Manifest V3 to inhibit ad blocking software due to its vested interest in the online advertising market.[324]Google cited performance issues associated with WebRequest, as well as its use in malicious extensions. In June 2019, it announced that it would increase the aforementioned cap from 30,000 to 150,000 entries to help quell concerns about limitations to filtering rules.[325][326][327][321]In 2021, theElectronic Frontier Foundation(EFF) issued a statement that Manifest V3 was "outright harmful to privacy efforts", as it would greatly limit the functionality of ad blocking extensions.[289] In December 2022, Google announced the transition would be paused "in order to address developer feedback and deliver better solutions to migration issues." In November 2023, Google announced it would resume the transition to Manifest V3; support for Manifest V2 extensions would be removed entirely from non-stable builds of Chrome beginning June 2024.[328][321]Other Chromium-based web browsers will adopt Manifest V3, includingMicrosoft Edge.[329]Manifest V3 support is being added to Mozilla Firefox's implementation of Chrome's extension API (WebExtensions) for compatibility reasons, but Mozilla has stated that its implementation would not contain limitations that affect privacy and content-blocking extensions, and that its implementation of V2 would not be deprecated.[323] In August 2024, a federal judge inWashington, D.C.ruled that Google maintained an illegal monopoly over search services.[330]In November 2024, the US Department of Justice (DOJ) demanded thatGooglesell Chrome to stop Google from maintaining its monopoly in online search.[331] Chrome overtook Firefox in November 2011, in worldwide usage. As of September 2022[update], according to StatCounter, Google Chrome had 67% worldwide desktop usage share, making it the most widely used web browser.[333] It was reported by StatCounter, a web analytics company, that for the single day of Sunday, March 18, 2012, Chrome was the most used web browser in the world for the first time. Chrome secured 32.7% of the global web browsing on that day, while Internet Explorer followed closely behind with 32.5%.[334] From May 14–21, 2012, Google Chrome was for the first time responsible for more Internet traffic than Microsoft's Internet Explorer, which long had held its spot as the most used web browser in the world.[335]According to StatCounter, 31.88% of web traffic was generated by Chrome for a sustained period of one week and 31.47% by Internet Explorer. Though Chrome had topped Internet Explorer for a single day's usage in the past, this was the first time it had led for one full week.[336] At the 2012 Google I/O developers' conference, Google claimed that there were 310 million active users of Chrome, almost double the number in 2011, which was stated as 160 million active users.[337] In June 2013, according to StatCounter, Chrome overtook Internet Explorer for the first time in the US.[338] In August 2013, Chrome was used by 43% of internet users worldwide. This study was done by Statista, which also noted that in North America, 36% of people use Chrome, the lowest in the world.[339] In December 2010, Google announced that to make it easier for businesses to use Chrome they would provide an official ChromeMSI package. For business use it is helpful to have full-fledged MSI packages that can be customized via transform files (.mst) – but the MSI provided with Chrome is only a very limitedMSI wrapperfitted around the normal installer, and many businesses find that this arrangement does not meet their needs.[341]The normal downloaded Chrome installer puts the browser in the user's local app data directory and provides invisible background updates, but the MSI package will allow installation at the system level, providing system administrators control over the update process[342]– it was formerly possible only when Chrome was installed usingGoogle Pack. Google also created group policy objects to fine-tune the behavior of Chrome in the business environment, for example by setting automatic updates intervals, disabling auto-updates, and configuring a home page.[343]Until version 24 the software is known not to be ready for enterprise deployments with roaming profiles or Terminal Server/Citrix environments.[344] In 2010, Google first started supporting Chrome in enterprise environments by providing an MSI wrapper around the Chrome installer. Google starting providing group policy objects, with more added each release,[345]and today there are more than 500 policies available to control Chrome's behavior in enterprise environments.[346]In 2016, Google launched Chrome Browser Enterprise Support, a paid service enabling IT admins access to Google experts to support their browser deployment.[347]In 2019, Google launchedChrome Browser Cloud Management, a dashboard that gives business IT managers the ability to control content accessibility, app usage and browser extensions installed on its deployed computers.[348] In September 2008, Google released a large portion of Chrome's source code as an open-source project called Chromium. This move enabled third-party developers to study the underlying source code and to help port the browser to the macOS and Linux operating systems. The Google-authored portion of Chromium is released under the permissiveBSD license.[349]Other portions of the source code are subject to a variety of open-source licenses.[350]Chromium is similar to Chrome, but lacks built-in automatic updates and a built-in Flash player, as well as Google branding and has a blue-colored logo instead of the multicolored Google logo.[351][352]Chromium does not implement user RLZ tracking.[300][295][353]Initially, the Google Chrome PDF viewer, PDFium, was excluded from Chromium, but was later made open-source in May 2014.[354][355]PDFium can be used to fill PDF forms.[356] It is possible to develop applications, extensions, and themes for Chrome. They are zipped in a .crx file and contain a manifest.json file that specifies basic information (such as version, name, description, privileges, etc.), and other files for the user interface (icons, popups, etc.). Google has an official developer's guide on how to create, develop, and publish projects.[357]Chrome has its own web store where users and developers can upload and download these applications and extensions.[358] As withMicrosoft Internet Explorer, the popularity of Google Chrome has led to the appearance ofmalwareabusing its name. In late 2015, anadwarereplica of Chrome named "eFast" appeared, which would usurp the Google Chrome installation and hijack file type associations to make shortcuts for common file types andcommunication protocolslink to itself, and inject advertisements into web pages. Its similar-looking icon was intended to deceive users.[359][360][361]	https://en.wikipedia.org/wiki/Google_Chrome
107.0.5045.11 (February 1, 2024; 15 months ago(2024-02-01)[2][3][4]) Operais a multi-platformweb browserdeveloped by its namesake companyOpera.[11][12][13]The current edition of the browser is based onChromium. Opera is available onWindows,macOS,Linux,Android, andiOS(SafariWebKitengine).[14][15]Opera offers two mobile versions, calledOpera Mobile[16]andOpera Mini.[17][18] Opera was released on Monday, April 10, 1995, making it one of the oldest desktop web browsers to exist. It wascommercial softwarefor its first ten years and had its own proprietary layout engine,Presto. In 2013, it switched from the Presto engine toChromium. In 2019, Opera introducedOpera GX, a browser marketed towards gamers, claiming to have better performance with a built-in ad and tracker blocker and also having aCPUandRAMusage limiter.[19]The browser has a feature called mods which made it possible to change the look of the browser with dynamic backgrounds, custom Opera logos, etc. In 2025, Opera releasedOpera Air, a more lightweight version of the browser designed with a focus on mindfulness and user well-being. It integrates standard browsing functionalities with tools aimed at reducing digital stress and enhancing productivity. In 1994,Jon Stephenson von TetzchnerandGeir Ivarsøystarted developing the Opera web browser while working atTelenor, aNorwegiantelecommunications company.[20][21] In 1995, they foundedOpera Software AS.[22]Opera was initially released on 10 April 1995, and then it was released publicly in 1996 with version 2.10,[23]which ran onMicrosoft Windows 95.[24]Development for mobile device platforms started in 1998.[25] Opera 4.0, released in 2000,[23]included a new cross-platform core that facilitated the creation of editions of Opera for multipleoperating systemsand platforms.[26] To this point, Opera wastrialwareand had to be purchased after the trial period. With version 5.0, released in 2000, Opera becamead-sponsored, displaying ads to users who had not paid for it.[27]Subsequent versions have given users the choice of seeingbanner adsor targeted text ads fromGoogle. With version 8.5, released in 2005, the ads were completely removed, and the browser's primary financial support came through revenue from Google (by contract, Opera's defaultsearch engine).[28] Among new features introduced in version 9.1, released in 2006, was fraud protection using technology fromGeoTrust, adigital certificateprovider, andPhishTank, an organization that tracks knownphishingweb sites.[29]This feature was further expanded in version 9.5, when GeoTrust was replaced withNetcraft, and malware protection from Haute Secure was added.[30] In 2006, Opera Software ASA was released as well asNintendo DS BrowserandInternet Channelfor Nintendo'sDSandWiigaming systems, respectively, which were Opera-based browsers.[31][32][33][34] A newJavaScriptengine, calledCarakan(after theJavanese alphabet), was introduced with version 10.50.[35]According to Opera Software, it made Opera 10.50 more than seven times faster inSunSpiderthan Opera 10.10.[36][37][38] On 16 December 2010, Opera 11 was released, featuringextensions,[39]tab stacking (where dragging one tab over another allowed creating a group of tabs), visual mouse gestures and changes to the address bar.[40]Opera 12 was released on 14 June 2012.[41] On 12 February 2013, Opera Software announced that it would drop its ownPrestolayout engine in favor ofWebKitas implemented by Google'sChromebrowser, using code from theChromiumproject. Opera Software planned as well to contribute code to WebKit.[42]On 3 April 2013, Google announced it would fork components from WebKit to form a new layout engine,Blink. That day, Opera Software confirmed it would follow Google in implementing Blink.[43] On 28 May 2013, a beta release of Opera 15 was made available,[44]the first version based on the Chromium project.[45][46]Many distinctive Opera features of the previous versions were dropped, and Opera Mail was separated into a standalone application derived from Opera 12.[47] In 2016, Opera was acquired by an investment group led by a Chinese consortium, the consortium included several Chinese companies such as Kunlun Tech andQihoo 360. On July 27, 2018, Opera Software wentpublicon theNASDAQstock exchange, raising $115 million in itsinitial public offering.[48]Opera began repurchasing its shares in 2022 following the closure of360 Security Technology Inc.that year.[49][50] In January 2017, the source code of Opera 12.15, one of the last few versions still based on the Presto layout engine, was leaked.[citation needed] To demonstrate how radically different a browser could look, Opera Neon, dubbed a "concept browser", was released in January 2017.PC Worldcompared it to demo models that automakers and hardware vendors release to show their visions of the future. Instead of aSpeed Dial Browsingfeature it displays the frequently accessed websites in resemblance to adesktopwith computer icons scattered over it in an artistic formation.[51][52] On 10 May 2017, Opera 45 was released. Notably this was the last version of the browser compatible with32-bit Linuxdistributions, with later versions requiring a64-bit Linuxdistribution. This version, inspired by the previous Opera Neon design, was called "Opera Reborn" and which redoes parts of the user interface, such as adding light and dark modes, and integrates the messenger applicationsFacebook Messenger,WhatsApp, andTelegram. Additionally, new ad-blocking settings were added along with security changes.[53] On 4 January 2018, Opera 50 was released. This version updated the browser to utilize the built-in ad blocker to providecryptocurrency miningprotection that stops sites from running scripts that attempt to use the CPU to mine cryptocurrency. Additionally the browser addedChromecastsupport, VR support enhancements, saving pages as PDFs, and improved VPN performance with region-based locations rather than country-based.[54] On 9 April 2019, Opera 60 was released. This version, codenamed Reborn 3, focused on moving the browser towards a more minimal design, further improving the free VPN service, and was marketed as being the "World's firstWeb3ready browser", as it included out of the box integrations with blockchain and cryptocurrency applications.[55] On 21 May 2019, Opera GX is announced and opened for early access. The only information available in this announcement is that the browser would be a special version of the browser aimed at those who play games. The early-access program was opened on 11 June 2019.[56][57] On 24 June 2021, Opera 77, codenamed Opera R5, was released. As one of the larger updates to the browser, it added more music streaming services in the sidebar, integrating native support forApple Music,Spotify,YouTube Music,Tidal,SoundCloud, andGaana. The "Pinboards" feature was also added, letting users create a shareable collection of websites, images, links, and notes in a visual form. A video popout feature was also added for video conferencing, which happens automatically when switching tabs, popping out of the window when navigating away and popping back in when navigating back.[58]Later, in Opera 83 released on 19 January 2022, this feature would be implemented for all video players, not just video conferencing platforms.[59] On 31 Jan 2023, Opera announced that given the discontinuation of support forWindows 7and8.1byMicrosoft,Chromium based browsersare also ending support, so Opera will no longer get updates on those versions, but older versions will continue to function on those versions of Windows.[60] On 22 March 2023, Opera and Opera GX incorporated features with AI-powered tools. These features include AI Prompts that are suggested to the user, and sidebar access toChatGPTand ChatSonic. The prompts show up on sites that contain content like articles, offering to shorten the text or summarize them.[61] On 20 June 2023, Opera launched Opera 100, codenamed Opera One, a version of the browser built from the ground up around AI which was unveiled on 25 April 2023. This browser includes a native AI called Aria, aGPT-basedAI engine that was developed collaboratively withOpenAIthat sifts through web information, generates text and code, and more. Tab islands were also introduced, allowing browser tabs to be grouped together, bookmarked, collapsed, and more. Major UI changes were made, and a Multithreaded Compositor was introduced, allowing the browser to function and render in animations much smoother than it was previously capable.[62][63][64]In February 2025, the platform launched preview of its Browser Operator, an AI assistant that handles browsing tasks for users.[65] Opera has originated features later adopted by other web browsers, including: Speed Dial, pop-up blocking, reopening recently closed pages,private browsing, andtabbed browsing.[66][67]Additional features include a built-in screenshot tool, Snapshot, which also includes an image-markup tool;[68]built-in ad blockers, and tracking blockers.[69] Opera's desktop browser includes access to social media messaging appsWhatsApp,Discord,Telegram,Facebook Messenger,X(Formerly Twitter),Instagram,TikTok, andVK.[70][71] Opera includes abookmarksbar and adownload manager. It also has "Speed Dial" which allows the user to add an unlimited number of pages shown inthumbnailform in a page displayed when a new tab is opened.[66][67][72] Opera was one of the first browsers to supportCascading Style Sheets(CSS) in 1998.[73] Opera Turbo, a feature that compresses requested web pages (exceptHTTPSpages) before sending them to the users,[74]is no longer available on the desktop browser. Opera Turbo is available inOpera Mini, the mobile browser.[75] One security feature is the option to delete private data, such asHTTP cookies, browsing history, items incacheand passwords with the click of a button.[76] When visiting a site, Opera displays a security badge in the address bar which shows details about the website, including security certificates.[77]Opera'sfraudandmalwareprotection warns the user about suspicious web pages and is enabled by default. It checks the requested page against several databases of knownphishingand malware websites, called blacklists.[77] In 2016, a free virtual private network (VPN) service was implemented in the browser.[78]Opera said that this would allow encrypted access to websites otherwise blocked, and provide security on public WiFi networks.[79][80]Security experts atDeloitteinspected Opera VPN's technical infrastructure to ensure users' data linked to their activities is never stored or logged as stated on Opera's no-log policy.[81] In 2018, a built-incryptocurrency walletfor the Opera browser was released,[82]with an announcement that Opera would be the first browser with such a feature.[83]On 13 December 2018, Opera released a video showing many decentralized applications likeCryptokittiesrunning on the Android version of the Opera Web Browser.[84] In March 2020, Opera updated itsAndroidbrowser to access.cryptodomains (IPFS), making it the first browser to be able to support adomain name system(DNS) which is not part of the traditional DNS directly without the need of a plugin or add-on.[85]This was through a collaboration with aSan Francisco–based startup, Unstoppable Domains.[86][87]In 2021 iOS and desktop versions have support forIPFS.[88] In January 2022, Opera introduced Opera Crypto Browser into public beta, combining a non-custodial wallet with a dedicated browser for blockchain-enabled services and Web3 technologies.[89]On 14 April 2022, Opera launched its Crypto Browser available on iOS devices.[90]As of 2023, Opera Crypto Browser has been discontinued, with the browser's features being integrated into Opera and Opera GX. In 2023, Opera added anAI chatbotcalled Aria into the browser. This is powered by Opera's Composer AI engine and connects toOpenAI'sGPTmodel. Opera GX is a gaming-oriented alternative to Opera. The browser was announced on 21 May 2019 and released inearly accessfor Windows on 11 June 2019, duringE3 2019. The macOS version was released in December of the same year.[91][92][56] Opera GX adds features geared toward gamers and other audiences, with the regular Opera browser features included. The limiter allows users to limit network, CPU, and memory usage to preserve system resources. GX Cleaner is a tool that is said to allow users to clearcache,cookies, and other unwanted files etc. The browser also adds integrations with other websites such asTwitch,Discord,Twitter, andInstagram. The browser also has a built-in page called the GX Corner, which combines gaming-related releases, deals, and news articles.[93][94][95] On 5 September 2019, Opera won aRed Dotaward in the Interface and User Experience Design category for Opera GX. Around the same time, Opera GX also marked its first one million downloads.[96] On 20 May 2021, a mobile version of Opera GX was released oniOSandAndroid.[97] On 12 November of the same year, Opera GX Mobile was awarded anotherRed Dotaward in both the Apps category and theInterfaceandUser Experience Design: Mobile UIs category.[98] On August 19, 2023, Opera GX introduced aVTubernamed GX Aura for their social media accounts; she later became the mascot for the browser.[99][100] On November 28 of the same year, Opera GX launched a new advertising campaign titled "Bury Boring", featuring actor and comedianEric Andrésmashing and burying computers not using Opera GX.[101][102]The advertising campaign also included a splash screen whenever the browser is launched during the campaign with slashing sounds and André yelling out the browser's name. While well received by many of its user base, many criticized Opera GX's actions.[citation needed] Opera Air is a mindfulness-oriented browser alternative to Opera and Opera GX. The browser was launched on February 4, 2025. Opera Air includes built-in features such as breathing exercises, meditation, binaural beats, and neck stretches to promote relaxation and mindfulness. Also the browser features a clean and calming interface with a "frosted glass" appearance to create a sense of tranquility.[103]And offers "Boosts" of calming or stimulating music to enhance the browsing experience and support productivity or relaxation.[104] Opera Software uses a release cycle consisting of three "streams", corresponding to phases of development, that can be downloaded and installed independently of each other: "developer", "beta", and "stable". New features are first introduced in thedeveloper build, then, depending on user feedback, may progress to thebeta versionand eventually bereleased.[105] The developer stream allows early testing of new features, mainly targeting developers, extension creators, and early adopters. Opera developer is not intended for everyday browsing as it is unstable and is prone to failure or crashing, but it enables advanced users to try out new features that are still under development, without affecting their normal installation of the browser. New versions of the browser are released frequently, usually a few times a week.[106] The beta stream, formerly known as "Opera Next", is afeature completepackage, allowing stability and quality to mature before the final release. A new version is released every couple of weeks.[107]Both streams can be installed alongside the official release without interference. Each has a different icon to help the user distinguish between the variants.[108] In 2005,Adobe Systemsintegrated Opera's rendering engine, Presto, into itsAdobe Creative Suiteapplications. Opera technology was employed inAdobe GoLive,Adobe Photoshop,Adobe Dreamweaver, and other components of theAdobe Creative Suite.[110][111]Opera's layout engine is also found in Virtual Mechanics SiteSpinner Pro.[112]The Internet Channel is a version of the Opera 9 web browser for use on theNintendo Wiicreated by Opera Software and Nintendo.[113]Opera Software is also implemented in theNintendo DS BrowserandNintendo DSi Browserfor Nintendo's handheld systems. Opera is the fifth most popular web-browser.[114][115]As of April 2021, Opera's offerings had over 320 million active users.[116] The Opera browser has been listed as a "tried and tested direct alternative to Chrome".[117]It scores close to Chrome on the HTML5test, which scores browsers' compatibility with different web standards.[115] Versions with the Presto layout engine have been positively reviewed,[118][119][120]although they have been criticized for website compatibility issues.[121][122]Because of this issue, Opera 8.01 and higher had included workarounds to help certain popular but problematic web sites display properly.[123][124] Versions with the Blink layout engine have been criticized by some users for missing features such asUIcustomization, and for abandoning Opera Software's own Presto layout engine.[125][126][127][128]Despite that, versions with the Blink layout engine have been noted for being fast and stable, for handling the latest web standards and for having a better website compatibility and a modern-style user interface.[129][130][131] Opera browser platform variants: Related other browsers: Related topics:	https://en.wikipedia.org/wiki/Opera_(web_browser)
HTTP Public Key Pinning(HPKP) is an obsoleteInternet securitymechanism delivered via anHTTPheaderwhich allowsHTTPSwebsites to resistimpersonationby attackers using misissued or otherwise fraudulentdigital certificates.[1]A server uses it to deliver to theclient(e.g. aweb browser) a set of hashes ofpublic keysthat must appear in the certificate chain of future connections to the samedomain name. For example, attackers might compromise acertificate authority, and then mis-issue certificates for aweb origin. To combat this risk, the HTTPS web server serves a list of “pinned” public key hashes valid for a given time; on subsequent connections, during that validity time, clients expect the server to use one or more of those public keys in its certificate chain. If it does not, an error message is shown, which cannot be (easily) bypassed by the user. The technique does not pin certificates, butpublic keyhashes. This means that one can use thekey pairto get a certificate from any certificate authority, when one has access to the private key. Also the user can pin public keys ofrootorintermediate certificates(created by certificate authorities), restricting site to certificates issued by the said certificate authority. Due to HPKP mechanism complexity and possibility of accidental misuse (potentially causing a lockout condition by system administrators), in 2017 browsers deprecated HPKP and in 2018 removed its support in favor ofCertificate Transparency.[2][3] The server communicates the HPKP policy to the user agent via anHTTPresponse header field namedPublic-Key-Pins(orPublic-Key-Pins-Report-Onlyfor reporting-only purposes). The HPKP policy specifieshashesof the subject public key info of one of the certificates in the website's authentic X.509public key certificatechain (and at least one backup key) inpin-sha256directives, and a period of time during which the user agent shall enforce public key pinning inmax-agedirective, optionalincludeSubDomainsdirective to include all subdomains (of the domain that sent the header) in pinning policy and optionalreport-uridirective with URL where to send pinning violation reports. At least one of the public keys of the certificates in the certificate chain needs to match a pinned public key in order for the chain to be considered valid by the user agent. At the time of publishing,RFC7469only allowed theSHA-256hash algorithm. (Appendix A. of RFC 7469mentions some tools and required arguments that can be used to produce hashes for HPKP policies.) A website operator can choose to either pin theroot certificatepublic key of a particular root certificate authority, allowing only that certificate authority (and all intermediate authorities signed by its key) to issue valid certificates for the website's domain, and/or to pin the key(s) of one or more intermediate issuing certificates, or to pin the end-entity public key. At least one backup key must be pinned, in case the current pinned key needs to be replaced. The HPKP is not valid without this backup key (a backup key is defined as a public key not present in the current certificate chain).[4] HPKP is standardized inRFC7469.[1]It expands on staticcertificate pinning, which hardcodes public key hashes of well-known websites or services within web browsers and applications.[5] Most browsers disable pinning forcertificate chainswith privateroot certificatesto enable various corporatecontent inspectionscanners[6]and web debugging tools (such asmitmproxyorFiddler). The RFC 7469 standard recommends disabling pinning violation reports for "user-defined" root certificates, where it is "acceptable" for the browser to disable pin validation.[7] If the user agent performs pin validation and fails to find a valid SPKI fingerprint in the served certificate chain, it will POST a JSON formattedviolation reportto the host specified in thereport-uridirective containing details of the violation. This URI may be served viaHTTPorHTTPS; however, the user agent cannot send HPKP violation reports to an HTTPS URI in the same domain as the domain for which it is reporting the violation. Hosts may either use HTTP for thereport-uri, use an alternative domain, or use a reporting service.[8] Some browsers also support thePublic-Key-Pins-Report-Only, which only triggers this reporting while not showing an error to the user. During its peak adoption, HPKP was reported to be used by 3,500 of top 1 million internet sites, a figure that declined to 650 around the end of 2019.[9] Criticism and concern revolved around malicious or human error scenarios known as HPKP Suicide and RansomPKP.[10]In such scenarios, a website owner would have their ability to publish new contents to their domain severely hampered by either losing access to their own keys or having new keys announced by a malicious attacker.	https://en.wikipedia.org/wiki/HTTP_Public_Key_Pinning
Acountermeasureis a measure or action taken to counter or offset another one. As a general concept, it implies precision and is any technological or tactical solution or system designed to prevent an undesirable outcome in the process. The first known use of the term according to theMerriam-Websterdictionary was in 1923.[1] Countermeasures can refer to the following disciplinary spectrum: Defense countermeasures are often divided into "active" and "passive". "Active" countermeasures mean the system user or the defender takes an active position because the incoming incident is known so the system takes active approaches to deal with such possible damage. Such an approach may include setting up a security method for the incident or actively trying to stop or intersect such damage. "Passive" countermeasures mean the system is not aware of the incoming incident or potential security issues. To mitigate the result of any security issues, the system sets up a set of passive approach which only activates when the system encounters security problems. Usually, "Passive" countermeasures include: This includes information on security or defensive technology, usually a way to protect the system. For example, security software or firewall could also be thought of as an approach to defensive technology. These methods detect potential security issues and report back to the system or protect the system when the system is under a certain threat. This means the system has damage control about the possible outcome of the security problem. For example, the system might have a backup in a remote area so even if the current system is damaged, the system could switch to the remote backup and works seamlessly. This means the system sets up a security approach to separate the core of the system. This approach is commonly used in a modern server network, where the server user has to go through ajump serverto access the core server. The jump server works as a fortification to separate the core server and the outside, which the core server sometimes is not connected to the internet and only connects to the local network, so the user needs to access the jump server to access the core server	https://en.wikipedia.org/wiki/Countermeasure
TheCommon Vulnerability Scoring System(CVSS) is atechnical standardfor assessing the severity ofvulnerabilitiesin computing systems. Scores are calculated based on a formula with severalmetricsthat approximate ease and impact of an exploit. Scores range from 0 to 10, with 10 being the most severe. While many use only the CVSS Base score for determining severity, temporal and environmental scores also exist, to factor in availability of mitigations and how widespread vulnerable systems are within an organization, respectively. The current version of CVSS (CVSSv4.0) was released in November 2023.[1] CVSS is not intended to be used as a method forpatch managementprioritization, but is used like that regardless.[2]A more effective approach is to integrate CVSS with predictive models like the Exploit Prediction Scoring System (EPSS), which helps prioritize remediation efforts based on the likelihood of real-world exploitation.[3] Research by theNational Infrastructure Advisory Council(NIAC) in 2003/2004 led to the launch of CVSS version 1 (CVSSv1) in February 2005,[4]with the goal of being "designed to provide open and universally standard severity ratings of software vulnerabilities". This initial draft had not been subject to peer review or review by other organizations. In April 2005, NIAC selected the Forum of Incident Response and Security Teams (FIRST) to become the custodian of CVSS for future development.[5][6] Feedback from vendors using CVSSv1 in production suggested there were "significant issues with the initial draft of CVSS". Work on CVSS version 2 (CVSSv2) began in April 2005 with the final specification being launched in June 2007.[7] Further feedback resulted in work beginning on CVSS version 3[8]in 2012, ending with CVSSv3.0 being released in June 2015.[9][4] The CVSS assessment measures three areas of concern: A numerical score is generated for each of these metric groups. A vector string (or simply "vector" in CVSSv2) represents the values of all the metrics as a block of text. Complete documentation for CVSSv2 is available from FIRST.[10]A summary is provided below. The access vector (AV) shows how a vulnerability may be exploited. The access complexity (AC) metric describes how easy or difficult it is to exploit the discovered vulnerability. The authentication (Au) metric describes the number of times that an attacker must authenticate to a target to exploit it. It does not include (for example) authentication to a network in order to gain access. For locally exploitable vulnerabilities, this value should only be set to Single or Multiple if further authentication is required after initial access. The confidentiality (C) metric describes the impact on the confidentiality of data processed by the system. The Integrity (I) metric describes the impact on the integrity of the exploited system. The availability (A) metric describes the impact on the availability of the target system. Attacks that consume network bandwidth, processor cycles, memory, or any other resources affect the availability of a system. These six metrics are used to calculate the exploitability and impact sub-scores of the vulnerability. These sub-scores are used to calculate the overall base score. Exploitability=20×AccessVector×AccessComplexity×Authentication{\displaystyle {\textsf {Exploitability}}=20\times {\textsf {AccessVector}}\times {\textsf {AccessComplexity}}\times {\textsf {Authentication}}} Impact=10.41×(1−(1−ConfImpact)×(1−IntegImpact)×(1−AvailImpact)){\displaystyle {\textsf {Impact}}=10.41\times (1-(1-{\textsf {ConfImpact}})\times (1-{\textsf {IntegImpact}})\times (1-{\textsf {AvailImpact}}))} f(Impact)={0,ifImpact= 01.176,otherwise{\displaystyle f({\textsf {Impact}})={\begin{cases}0,&{\text{if }}{\textsf {Impact}}{\text{ = 0}}\\1.176,&{\text{otherwise }}\end{cases}}} BaseScore=roundTo1Decimal(((0.6×Impact)+(0.4×Exploitability)−1.5)×f(Impact)){\displaystyle {\textsf {BaseScore}}={\textsf {roundTo1Decimal}}(((0.6\times {\textsf {Impact}})+(0.4\times {\textsf {Exploitability}})-1.5)\times f({\textsf {Impact}}))} The metrics are concatenated to produce the CVSS Vector for the vulnerability. Abuffer overflowvulnerability affects web server software that allows a remote user to gain partial control of the system, including the ability to cause it to shut down: This would give an exploitability sub-score of 10, and an impact sub-score of 8.5, giving an overall base score of 9.0. The vector for the base score in this case would be AV:N/AC:L/Au:N/C:P/I:P/A:C. The score and vector are normally presented together to allow the recipient to fully understand the nature of the vulnerability and to calculate their own environmental score if necessary. The value of temporal metrics change over the lifetime of the vulnerability, as exploits are developed, disclosed and automated and as mitigations and fixes are made available. The exploitability (E) metric describes the current state of exploitation techniques or automated exploitation code. The remediation level (RL) of a vulnerability allows the temporal score of a vulnerability to decrease as mitigations and official fixes are made available. The report confidence (RC) of a vulnerability measures the level of confidence in the existence of the vulnerability and also the credibility of the technical details of the vulnerability. These three metrics are used in conjunction with the base score that has already been calculated to produce the temporal score for the vulnerability with its associated vector. The formula used to calculate the temporal score is: TemporalScore=roundTo1Decimal(BaseScore×Exploitability×RemediationLevel×ReportConfidence){\displaystyle {\textsf {TemporalScore}}={\textsf {roundTo1Decimal}}({\textsf {BaseScore}}\times {\textsf {Exploitability}}\times {\textsf {RemediationLevel}}\times {\textsf {ReportConfidence}})} To continue with the example above, if the vendor was first informed of the vulnerability by a posting of proof-of-concept code to a mailing list, the initial temporal score would be calculated using the values shown below: This would give a temporal score of 7.3, with a temporal vector of E:P/RL:U/RC:UC (or a full vector of AV:N/AC:L/Au:N/C:P/I:P/A:C/E:P/RL:U/RC:UC). If the vendor then confirms the vulnerability, then the score rises to 8.1, with a temporal vector of E:P/RL:U/RC:C A temporary fix from the vendor would reduce the score back to 7.3 (E:P/RL:T/RC:C), while an official fix would reduce it further to 7.0 (E:P/RL:O/RC:C). As it is not possible to be confident that every affected system has been fixed or patched, the temporal score cannot reduce below a certain level based on the vendor's actions, and may increase if an automated exploit for the vulnerability is developed. The environmental metrics use the base and current temporal score to assess the severity of a vulnerability in the context of the way that the vulnerable product or software is deployed. This measure is calculated subjectively, typically by affected parties. The collateral damage potential (CDP) metric measures the potential loss or impact on either physical assets such as equipment (and lives), or the financial impact upon the affected organisation if the vulnerability is exploited. The target distribution (TD) metric measures the proportion of vulnerable systems in the environment. Three further metrics assess the specific security requirements for confidentiality (CR), integrity (IR) and availability (AR), allowing the environmental score to be fine-tuned according to the users' environment. The five environmental metrics are used in conjunction with the previously assessed base and temporal metrics to calculate the environmental score and to produce the associated environmental vector. AdjustedImpact=min(10,10.41×(1−(1−ConfImpact×ConfReq)×(1−IntegImpact×IntegReq)×(1−AvailImpact×AvailReq))){\displaystyle {\textsf {AdjustedImpact}}=\min(10,10.41\times (1-(1-{\textsf {ConfImpact}}\times {\textsf {ConfReq}})\times (1-{\textsf {IntegImpact}}\times {\textsf {IntegReq}})\times (1-{\textsf {AvailImpact}}\times {\textsf {AvailReq}})))} AdjustedTemporal=TemporalScorerecomputed with theBaseScoresImpactsub-equation replaced with theAdjustedImpactequation{\displaystyle {\textsf {AdjustedTemporal}}={\textsf {TemporalScore}}{\text{ recomputed with the }}{\textsf {BaseScore}}{\text{s }}{\textsf {Impact}}{\text{ sub-equation replaced with the }}{\textsf {AdjustedImpact}}{\text{ equation}}} EnvironmentalScore=roundTo1Decimal((AdjustedTemporal+(10−AdjustedTemporal)×CollateralDamagePotential)×TargetDistribution){\displaystyle {\textsf {EnvironmentalScore}}={\textsf {roundTo1Decimal}}(({\textsf {AdjustedTemporal}}+(10-{\textsf {AdjustedTemporal}})\times {\textsf {CollateralDamagePotential}})\times {\textsf {TargetDistribution}})} If the aforementioned vulnerable web server were used by a bank to provide online banking services, and a temporary fix was available from the vendor, then the environmental score could be assessed as: This would give an environmental score of 8.2, and an environmental vector of CDP:MH/TD:H/CR:H/IR:H/AR:L. This score is within the range 7.0-10.0, and therefore constitutes a critical vulnerability in the context of the affected bank's business. Several vendors and organizations expressed dissatisfaction with CVSSv2. Risk Based Security, which manages theOpen Source Vulnerability Database, and theOpen Security Foundationjointly published a public letter to FIRST regarding the shortcomings and failures of CVSSv2.[11]The authors cited a lack of granularity in several metrics, which results in CVSS vectors and scores that do not properly distinguish vulnerabilities of different type and risk profiles. The CVSS scoring system was also noted as requiring too much knowledge of the exact impact of the vulnerability. Oracleintroduced the new metric value of "Partial+" for Confidentiality, Integrity, and Availability, to fill perceived gaps in the description between Partial and Complete in the official CVSS specifications.[12] To address some of these criticisms, development of CVSS version 3 was started in 2012. The final specification was named CVSSv3.0 and released in June 2015. In addition to a Specification Document, a User Guide and Examples document were also released.[13] Several metrics were changed, added, and removed. The numerical formulas were updated to incorporate the new metrics while retaining the existing scoring range of 0-10. Textual severity ratings of None (0), Low (0.1-3.9), Medium (4.0-6.9), High (7.0-8.9), and Critical (9.0-10.0)[14]were defined, similar to the categories NVD defined for CVSSv2 that were not part of that standard.[15] In the Base vector, the new metrics User Interaction (UI) and Privileges Required (PR) were added to help distinguish vulnerabilities that required user interaction or user or administrator privileges to be exploited. Previously, these concepts were part of the Access Vector metric of CVSSv2. UI can take the values None or Required; attacks that do not require logging in as a user are considered more severe. PR can take the values None, Low, or High; similarly, attacks requiring fewer privileges are more severe. The Base vector also saw the introduction of the new Scope (S) metric, which was designed to make clear which vulnerabilities may be exploited and then used to attack other parts of a system or network. These new metrics allow the Base vector to more clearly express the type of vulnerability being evaluated. The Confidentiality, Integrity, and Availability (C, I, A) metrics were updated to have scores consisting of None, Low, or High, rather than the None, Partial, and Complete of CVSSv2. This allows more flexibility in determining the impact of a vulnerability on CIA metrics. Access Complexity was renamed Attack Complexity (AC) to make clear that access privileges were moved to a separate metric. This metric now describes how repeatable exploit of this vulnerability may be; AC is High if the attacker requires perfect timing or other circumstances (other than user interaction, which is also a separate metric) which may not be easily duplicated on future attempts. Attack Vector (AV) saw the inclusion of a new metric value of Physical (P), to describe vulnerabilities that require physical access to the device or system to perform. The Temporal metrics were essentially unchanged from CVSSv2. The Environmental metrics of CVSSv2 were completely removed and replaced with essentially a second Base score, known as the Modified vector. The Modified Base is intended to reflect differences within an organization or company compared to the world as a whole. New metrics to capture the importance of Confidentiality, Integrity, and Availability to a specific environment were added. In a blog post in September 2015, theCERT Coordination Centerdiscussed limitations of CVSSv2 and CVSSv3.0 for use in scoring vulnerabilities in emerging technology systems such as theInternet of Things.[16] A minor update to CVSS was released on June 17, 2019. The goal of CVSSv3.1 was to clarify and improve upon the existing CVSSv3.0 standard without introducing new metrics or metric values, allowing for frictionless adoption of the new standard by both scoring providers and scoring consumers alike. Usability was a prime consideration when making improvements to the CVSS standard. Several changes being made in CVSSv3.1 are to improve the clarity of concepts introduced in CVSSv3.0, and thereby improve the overall ease of use of the standard. FIRST has used input from industry subject-matter experts to continue to enhance and refine CVSS to be more and more applicable to the vulnerabilities, products, and platforms being developed over the past 15 years and beyond. The primary goal of CVSS is to provide a deterministic and repeatable way to score the severity of a vulnerability across many different constituencies, allowing consumers of CVSS to use this score as input to a larger decision matrix of risk, remediation, and mitigation specific to their particular environment and risk tolerance. Updates to the CVSSv3.1 specification include clarification of the definitions and explanation of existing base metrics such as Attack Vector, Privileges Required, Scope, and Security Requirements. A new standard method of extending CVSS, called the CVSS Extensions Framework, was also defined, allowing a scoring provider to include additional metrics and metric groups while retaining the official Base, Temporal, and Environmental Metrics. The additional metrics allow industry sectors such as privacy, safety, automotive, healthcare, etc., to score factors that are outside the core CVSS standard. Finally, the CVSS Glossary of Terms has been expanded and refined to cover all terms used throughout the CVSSv3.1 documentation. Version 4.0 was officially released in November 2023,[1]and is available at FIRST.[17]Among several clarifications, the most notable changes are the new base metricAttack Requirementswhich complement the metricAttack Complexitywith an assessment what conditions at the target side are needed to exploit a vulnerability. Further, theImpactmetrics are split into impact on the vulnerable system itself and impact on subsequent systems (this replaces theScopemetric from prior versions). The base metrics are now as follows. Additionally to these base metrics, there are optional metrics regarding public availability of an exploit, environment specific thread modelling, system recovery, and others. Assume there is anSQL-Injectionin an online web shop. The database user of the online shop software only has read access to the database. Further the injection is in a view of the shop which is only visible to registered customers. The CVSS 4.0 base vector is as follows. This results in the vectorAV:N/AC:L/AT:N/PR:L/UI:N/VC:H/VI:N/VA:L/SC:N/SI:N/SA:L Versions of CVSS have been adopted as the primary method for quantifying the severity of vulnerabilities by a wide range of organizations and companies, including:	https://en.wikipedia.org/wiki/Common_Vulnerability_Scoring_System
In the field ofcomputer security, independent researchers often discover flaws in software that can be abused to cause unintended behaviour; these flaws are calledvulnerabilities. The process by which the analysis of these vulnerabilities is shared with third parties is the subject of much debate, and is referred to as the researcher'sdisclosure policy.Full disclosureis the practice of publishing analysis of software vulnerabilities as early as possible, making the data accessible to everyone without restriction. The primary purpose of widely disseminating information about vulnerabilities is so that potential victims are as knowledgeable as those who attack them.[1] In his 2007 essay on the topic,Bruce Schneierstated "Full disclosure – the practice of making the details of security vulnerabilities public – is a damned good idea. Public scrutiny is the only reliable way to improve security, while secrecy only makes us less secure."[2]Leonard Rose, co-creator of anelectronic mailing listthat has supersededbugtraqto become the de facto forum for disseminating advisories, explains "We don't believe insecurity by obscurity, and as far as we know, full disclosure is the only way to ensure that everyone, not just the insiders, have access to the information we need."[3] The controversy around the public disclosure of sensitive information is not new. The issue of full disclosure was first raised in the context of locksmithing, in a 19th-century controversy regarding whether weaknesses in lock systems should be kept secret in the locksmithing community, or revealed to the public.[4]Today, there are three major disclosure policies under which most others can be categorized:[5]Non Disclosure,Coordinated Disclosure, and Full Disclosure. The major stakeholders in vulnerability research have their disclosure policies shaped by various motivations, it is not uncommon to observe campaigning, marketing or lobbying for their preferred policy to be adopted and chastising those who dissent. Many prominent security researchers favor full disclosure, whereas most vendors prefer coordinated disclosure. Non disclosure is generally favored by commercial exploit vendors andblackhat hackers.[6] Coordinated vulnerability disclosureis a policy under which researchers agree to report vulnerabilities to a coordinating authority, which then reports it to the vendor, tracks fixes and mitigations, and coordinates the disclosure of information with stakeholders including the public.[7][8]In some cases the coordinating authority is the vendor. The premise of coordinated disclosure is typically that nobody should be informed about a vulnerability until the software vendor says it is time.[9][10]While there are often exceptions or variations of this policy, distribution must initially be limited and vendors are given privileged access to nonpublic research.[11] The original name for this approach was "responsible disclosure", based on the essay by Microsoft Security Manager Scott Culp “It's Time to End Information Anarchy”[12](referring to full disclosure). Microsoft later called for the term to be phased out in favor of “Coordinated Vulnerability Disclosure” (CVD).[13][14] Although the reasoning varies, many practitioners argue that end-users cannot benefit from access to vulnerability information without guidance or patches from the vendor, so the risks of sharing research with malicious actors is too great for too little benefit. As Microsoft explain, "[Coordinated disclosure] serves everyone's best interests by ensuring that customers receive comprehensive, high-quality updates for security vulnerabilities but are not exposed to malicious attacks while the update is being developed."[14] To prevent vendors to indefinitely delaying the disclosure, a common practice in the security industry, pioneered by Google,[15]is to publish all the details of vulnerabilities after a deadline, usually 90 or 120[16]days reduced to 7 days if the vulnerability isunder active exploitation.[17] Full disclosure is the policy of publishing information on vulnerabilities without restriction as early as possible, making the information accessible to the general public without restriction. In general, proponents of full disclosure believe that the benefits of freely available vulnerability research outweigh the risks, whereas opponents prefer to limit the distribution. The free availability of vulnerability information allows users and administrators to understand and react to vulnerabilities in their systems, and allows customers to pressure vendors to fix vulnerabilities that vendors may otherwise feel no incentive to solve. There are some fundamental problems with coordinated disclosure that full disclosure can resolve. Discovery of a specific flaw or vulnerability is not a mutually exclusive event, multiple researchers with differing motivations can and do discover the same flaws independently. There is no standard way to make vulnerability information available to the public, researchers often use mailing lists dedicated to the topic, academic papers or industry conferences. Non disclosure is the policy that vulnerability information should not be shared, or should only be shared under non-disclosure agreement (either contractually or informally). Common proponents of non-disclosure include commercial exploit vendors, researchers who intend to exploit the flaws they find,[5]and proponents ofsecurity through obscurity. In 2009,Charlie Miller, Dino Dai Zovi andAlexander Sotirovannounced at the CanSecWest conference the "No More Free Bugs" campaign, arguing that companies are profiting and taking advantage of security researchers by not paying them for disclosing bugs.[18]This announcement made it to the news and opened a broader debate about the problem and its associated incentives.[19][20] Researchers in favor of coordinated disclosure believe that users cannot make use of advanced knowledge of vulnerabilities without guidance from the vendor, and that the majority is best served by limiting distribution of vulnerability information. Advocates argue that low-skilled attackers can use this information to perform sophisticated attacks that would otherwise be beyond their ability, and the potential benefit does not outweigh the potential harm caused by malevolent actors. Only when the vendor has prepared guidance that even the most unsophisticated users can digest should the information be made public. This argument presupposes that vulnerability discovery is a mutually exclusive event, that only one person can discover a vulnerability. There are many examples of vulnerabilities being discovered simultaneously, often being exploited in secrecy before discovery by other researchers.[21]While there may exist users who cannot benefit from vulnerability information, full disclosure advocates believe this demonstrates a contempt for the intelligence of end users. While it's true that some users cannot benefit from vulnerability information, if they're concerned with the security of their networks they are in a position to hire an expert to assist them as you would hire a mechanic to help with a car. Non disclosure is typically used when a researcher intends to use knowledge of a vulnerability to attack computer systems operated by their enemies, or to trade knowledge of a vulnerability to a third party for profit, who will typically use it to attack their enemies. Researchers practicing non disclosure are generally not concerned with improving security or protecting networks. However, some proponents[who?]argue that they simply do not want to assist vendors, and claim no intent to harm others. While full and coordinated disclosure advocates declare similar goals and motivations, simply disagreeing on how best to achieve them, non disclosure is entirely incompatible.	https://en.wikipedia.org/wiki/Full_disclosure_(computer_security)
TheMetasploit Projectis acomputer securityproject that provides information aboutsecurity vulnerabilitiesand aids inpenetration testingandIDS signaturedevelopment. It is owned byBoston, Massachusetts-based security company,Rapid7. Its best-known sub-project is theopen-source[3]Metasploit Framework, a tool for developing and executingexploitcode against a remote target machine. Other important sub-projects include the Opcode Database,shellcodearchive and related research. The Metasploit Project includesanti-forensicand evasion tools, some of which are built into the Metasploit Framework. In various operating systems it comes pre installed. Metasploit was created byH. D. Moorein 2003 as a portable network tool usingPerl. By 2007, the Metasploit Framework had been completely rewritten inRuby. On October 21, 2009, the Metasploit Project announced[4]that it had been acquired by Rapid7, a security company that provides unified vulnerability management solutions. Like comparable commercial products such as Immunity's Canvas orCore Security Technologies' Core Impact, Metasploit can be used to test the vulnerability of computer systems or to break into remote systems. Like manyinformation securitytools, Metasploit can be used for both legitimate and unauthorized activities. Since the acquisition of the Metasploit Framework, Rapid7 has added anopen coreproprietary edition called Metasploit Pro.[5] Metasploit's emerging position as thede factoexploit development framework[6]led to the release of software vulnerability advisories often accompanied[7]by a third party Metasploit exploit module that highlights the exploitability, risk and remediation of that particular bug.[8][9]Metasploit 3.0 began to includefuzzingtools, used to discover software vulnerabilities, rather than just exploits for known bugs. This avenue can be seen with the integration of thelorconwireless (802.11) toolset into Metasploit 3.0 in November 2006. The basic steps for exploiting a system using the Framework include. This modular approach – allowing the combination of any exploit with any payload – is the major advantage of the Framework. It facilitates the tasks of attackers, exploit writers and payload writers. Metasploit runs on Unix (including Linux and macOS) and on Windows. The Metasploit Framework can be extended to use add-ons in multiple languages. To choose an exploit and payload, some information about the target system is needed, such as operating system version and installed network services. This information can be gleaned withport scanningandTCP/IP stack fingerprintingtools such asNmap.Vulnerability scannerssuch asNessus, andOpenVAScan detect target system vulnerabilities. Metasploit can import vulnerability scanner data and compare the identified vulnerabilities to existing exploit modules for accurate exploitation.[10] There are several interfaces for Metasploit available. The most popular are maintained by Rapid7 and Strategic Cyber LLC.[11] The Metasploit Framework is the freely available, open-source edition of the Metasploit Project. It provides tools for vulnerability assessment and exploit development including: The Metasploit Framework is implemented in Ruby and uses a modular software architecture.[12] In October 2010, Rapid7 added Metasploit Pro, an open-core commercial Metasploit edition for penetration testers. Metasploit Pro adds onto Metasploit Express with features such as Quick Start Wizards/MetaModules, building and managingsocial engineeringcampaigns, web application testing, an advanced Pro Console, dynamic payloads for anti-virus evasion, integration with Nexpose for ad-hoc vulnerability scans, and VPN pivoting. The edition was released in October 2011, and included a free, web-based user interface for Metasploit. Metasploit Community Edition was based on the commercial functionality of the paid-for editions with a reduced set of features, including network discovery, module browsing and manual exploitation. Metasploit Community was included in the main installer. On July 18, 2019, Rapid7 announced the end-of-sale of Metasploit Community Edition.[13]Existing users were able to continue using it until their license expired. The edition was released in April 2010, and was an open-core commercial edition for security teams who need to verify vulnerabilities. It offers a graphical user interface, It integrated nmap for discovery, and added smart brute-forcing as well as automated evidence collection. On June 4, 2019, Rapid7 discontinued Metasploit Express Edition.[14] Armitageis a graphical cyber attack management tool for the Metasploit Project that visualizes targets and recommends exploits. It is afree and open sourcenetwork securitytool notable for its contributions tored teamcollaboration allowing for shared sessions, data, and communication through a single Metasploit instance.[15] The latest release of Armitage was in 2015. Cobalt Strike is a collection of threat emulation tools provided by HelpSystems to work with the Metasploit Framework.[16]Cobalt Strike includes all features ofArmitageand adds post-exploitation tools, in addition to report generation features.[17] Metasploit currently has over 2074 exploits, organized under the following platforms:AIX,Android,BSD,BSDi,Cisco,Firefox,FreeBSD,HP-UX,Irix,Java,JavaScript,Linux,mainframe, multi (applicable to multiple platforms),NetBSD,NetWare,NodeJS,OpenBSD,macOS,PHP,Python,R,Ruby,Solaris,Unix, andWindows. Note thatApple iOSis based on FreeBSD, and some FreeBSD exploits may work, while most won't. Metasploit currently has over 592 payloads. Some of them are: The Metasploit Framework includes hundreds of auxiliary modules that can perform scanning, fuzzing, sniffing, and much more. There are three types of auxiliary modules namely scanners, admin and server modules. Metasploit Framework operates as an open-source project and accepts contributions from the community through GitHub.com pull requests.[18]Submissions are reviewed by a team consisting of both Rapid7 employees and senior external contributors. The majority of contributions add new modules, such as exploits or scanners.[19] List of original developers:	https://en.wikipedia.org/wiki/Metasploit
Amonth of bugsis a strategy used by security researchers to draw attention to the lax security procedures of commercial software corporations. Researchers have started such a project for software products where they believe corporations have shown themselves to be unresponsive and uncooperative to security alerts.Responsible disclosureis not working properly, and then find and disclose onesecurity vulnerabilityeach day for one month. The original "Month of Bugs" was theMonth of Browser Bugs(MoBB) run by security researcherH. D. Moore.[1] Subsequent similar projects include:	https://en.wikipedia.org/wiki/Month_of_Bugs
Vulnerability managementis the "cyclical practice of identifying, classifying, prioritizing, remediating, and mitigating"software vulnerabilities.[1]Vulnerability management is integral tocomputer securityandnetwork security, and must not be confused withvulnerability assessment.[2] Vulnerabilities can be discovered with avulnerability scanner, which analyzes a computer system in search of known vulnerabilities,[3]such asopen ports, insecure software configurations, and susceptibility tomalwareinfections. They may also be identified by consulting public sources, such as NVD, vendor specific security updates or subscribing to a commercial vulnerability alerting service. Unknown vulnerabilities, such as azero-day,[3]may be found withfuzz testing. Fuzzing is a cornerstone technique where random or semi-random input data is fed to programs to detect unexpected behavior. Tools such as AFL (American Fuzzy Lop) and libFuzzer automate this process, making it faster and more efficient. Fuzzy testing can identify certain kinds of vulnerabilities, such as abuffer overflowwith relevanttest cases. Similarly,static analysistools analyze source code orbinariesto identify potential vulnerabilities without executing the program.Symbolic execution, an advanced technique combining static anddynamic analysis, further aids in pinpointing vulnerabilities.[4]Such analysis can be facilitated bytest automation. In addition,antivirus softwarecapable ofheuristicanalysis may discover undocumented malware if it finds software behaving suspiciously (such as attempting to overwrite asystem file). Correcting vulnerabilities may variously involve theinstallationof apatch, a change in network security policy, reconfiguration of software, or educatingusersaboutsocial engineering. Project vulnerabilityis the project's susceptibility to being subject to negative events, the analysis of their impact, and the project's capability to cope with negative events.[5]Based on Systems Thinking,project systemic vulnerability managementtakes a holistic vision, and proposes the following process: Coping with negative events is done, in this model, through: Redundancyis a specific method to increase resistance and resilience in vulnerability management.[6] Antifragilityis a concept introduced byNassim Nicholas Talebto describe the capacity of systems to not only resist or recover from adverse events, but also to improve because of them. Antifragility is similar to the concept ofpositive complexityproposed by Stefan Morcov.	https://en.wikipedia.org/wiki/Vulnerability_management
w3af(Web Application Attack and Audit Framework) is anopen-sourceweb application security scanner. The project provides avulnerability scannerand exploitation tool for Web applications.[2]It provides information aboutsecurity vulnerabilitiesfor use inpenetration testingengagements. The scanner offers agraphical user interfaceand acommand-line interface.[3] w3af is divided into two main parts, thecoreand theplug-ins.[4]The core coordinates the process and provides features that are consumed by the plug-ins, which find the vulnerabilities and exploit them. The plug-ins are connected and share information with each other using a knowledge base. Plug-ins can be categorized as Discovery, Audit,Grep, Attack, Output, Mangle, Evasion or Bruteforce. w3af was started by Andres Riancho in March 2007, after many years of development by the community. In July 2010, w3af announced its sponsorship and partnership withRapid7. With Rapid7's sponsorship the project will be able to increase its development speed and keep growing in terms of users and contributors. Note: April 11, 2024https://www.w3af.orgis giving connection timed out failures. However, documentation is still accessible athttp://docs.w3af.org/en/latest/. Redirected to W4af:https://github.com/w4afthat is still in Alpha development	https://en.wikipedia.org/wiki/W3af
Thecute cat theory of digital activismis atheoryconcerningInternet activism,Web censorship, and "cute cats" (a term used for any low-value, but popular online activity) developed byEthan Zuckermanin 2008.[1][2]It posits that most people are not interested in activism; instead, they want to use thewebfor mundane activities, including surfing forpornographyandlolcats("cute cats").[3]The tools that they develop for that (such asFacebook,Flickr,Blogger,Twitter, and similar platforms) are very useful tosocial movementactivists because they may lack resources to develop dedicated tools themselves.[3]This, in turn, makes theactivistsmore immune to reprisals by governments than if they were using a dedicated activism platform, because shutting down a popular public platform provokes a larger public outcry than shutting down an obscure one.[3] Zuckerman states that "Web 1.0was invented to allow physicists to share research papers.Web 2.0was created to allow people to share pictures of cute cats."[3]Zuckerman says that if a tool has "cute cat" purposes, and is widely used for low-value purposes, it can be and likely is used for online activism, too.[3] If the government chooses to shut down such generic tools, it will hurt people's ability to "look at cute cats online", spreading dissent and encouraging the activists' cause.[2][3] According to Zuckerman,internet censorship in the People's Republic of China, which relies on its own, self-censored, Web 2.0 sites, is able to circumvent the cute-cat problem becausethe governmentis able to provide people with access to cute-cat content on domestic,self-censoredsites while blocking access to Western sites, which are less popular in China than in many other places worldwide.[3][4] "Sufficiently usable read/write platforms will attract porn and activists. If there's no porn, the tool doesn't work. If there are no activists, it doesn't work well," Zuckerman has stated.[3]	https://en.wikipedia.org/wiki/Cute_cat_theory_of_digital_activism
Copy protection, also known ascontent protection,copy preventionandcopy restriction, is any measure to enforcecopyrightby preventing the reproduction of software, films, music, and other media.[1] Copy protection is most commonly found onvideotapes,DVDs,Blu-ray discs,HD-DVDs,computer softwarediscs,video gamediscs and cartridges,audio CDsand someVCDs. It also may be incorporated into digitally distributed versions of media and software. Some methods of copy protection have also led to criticism because it caused inconvenience for paying consumers or secretly installed additional or unwanted software to detect copying activities on the consumer's computer. Making copy protection effective while protecting consumer rights remains a problem with media publication. Media corporations have always used the termcopy protection, but critics argue that the term tends to sway the public into identifying with the publishers, who favor restriction technologies, rather than with the users.[2]Copy preventionandcopy controlmay be more neutral terms. "Copy protection" is a misnomer for some systems, because any number of copies can be made from an original and all of these copies will work, but only in one computer, or only with onedongle, or only with another device that cannot be easily copied. The term is also often related to, and confused with, the concept ofdigital restrictions management. Digital restrictions management is a more general term because it includes all sorts of management of works, including copy restrictions. Copy restriction may include measures that are not digital. A more appropriate term may be "technological protection measures" (TPMs),[3]which is often defined as the use of technological tools in order to restrict the use or access to a work. Unauthorized copying and distribution accounted for $2.4billion per year in lost revenue in the United States alone in 1990,[4]and is assumed to be causing impact on revenues in themusicand thevideo game industry, leading to proposal of stricter copyright laws such asPIPA. Copy protection is most commonly found onvideotapes, DVDs, computer software discs, video game discs and cartridges, audio CDs and someVCDs. Many media formats are easy to copy using a machine, allowing consumers to distribute copies to their friends, a practice known as "casual copying". Companies publish works under copyright protection because they believe that the cost of implementing the copy protection will be less than the revenue produced by consumers who buy the product instead of acquiring it through casually copied media. Opponents of copy protection argue that people who obtain free copies only use what they can get for free and would not purchase their own copy if they were unable to obtain a free copy. Some even argue that free copies increase profit; people who receive a free copy of a music CD may then go and buy more of that band's music, which they would not have done otherwise. Some publishers have avoided copy-protecting their products on the theory that the resulting inconvenience to their users outweighs any benefit of frustrating "casual copying". From the perspective of the end user, copy protection is always a cost.DRMand license managers sometimes fail, are inconvenient to use, and may not afford the user all of thelegal useof the product they have purchased. The termcopy protectionrefers to the technology used to attempt to frustrate copying, and not to the legal remedies available to publishers or authors whose copyrights are violated. Software usage models range from node locking to floating licenses (where a fixed number licenses can be concurrently used across an enterprise), grid computing (where multiple computers function as one unit and so use a common license) and electronic licensing (where features can be purchased and activated online). The termlicense managementrefers to broad platforms which enable the specification, enforcement and tracking ofsoftware licenses. To safeguard copy protection and license management technologies themselves against tampering and hacking,software anti‑tampermethods are used. Floating licensesare also being referred to asIndirect Licenses, and are licenses that at the time they are issued, there is no actual user who will use them. That has some technical influence over some of their characteristics.Direct Licensesare issued after a certain user requires it. As an example, an activatedMicrosoftproduct, contains aDirect Licensewhich is locked to thePCwhere the product is installed. From business standpoint, on the other hand, some services now try to monetize on additional services other than the media content so users can have better experience than simply obtaining the copied product.[5] I heard these stories that people would invest months on devising copy-protection schemes and then in three hours some kid in San Diego figuring it out. From a technical standpoint, it seems impossible to completely prevent users from making copies of the media they purchase, as long as a "writer" is available that can write to blank media. All types of media require a "player"—a CD player, DVD player, videotape player, computer orvideo game console—which must be able to read the media in order to display it to a human. Logically, a player could be built that reads the media and then writes an exact copy of what was read to the same type of media.[citation needed] At a minimum, digital copy protection of non-interactive works is subject to theanalog hole: regardless of any digital restrictions, if music can be heard by the human ear, it can also be recorded (at the very least, with a microphone and tape recorder); if a film can be viewed by the human eye, it can also be recorded (at the very least, with a video camera and recorder). In practice, almost-perfect copies can typically be made by tapping into the analog output of a player (e.g. thespeakeroutput orheadphonejacks) and, once redigitized into an unprotected form, duplicated indefinitely. Copyingtext-basedcontent in this way is more tedious, but the same principle applies: if it can be printed or displayed, it can also be scanned andOCRed. With basic software and some patience, these techniques can be applied by a typical computer-literate user.[citation needed] Since these basic technical facts exist, it follows that a determined individual will definitely succeed in copying any media, given enough time and resources. Media publishers understand this; copy protection is not intended to stop professional operations involved in the unauthorized mass duplication of media, but rather to stop "casual copying".[citation needed] Copying of information goods which are downloaded (rather than being mass-duplicated as with physical media) can be inexpensively customized for each download, and thus restricted more effectively, in a process known as "traitor tracing". They can be encrypted in a fashion which is unique for each user's computer, and the decryption system can be madetamper-resistant.[citation needed] Copyright protection in content platforms also cause increased market concentration and a loss in aggregate welfare. According to research on the European Directive on copyright in the Digital Single Market on platform competition, only users of large platforms will be allowed to upload content if the content is sufficiently valuable and network effects are strong.[7] Copy protection for computer software, especially for games, has been a longcat-and-mousestruggle between publishers andcrackers. These were (and are) programmers who defeated copy protection on software as a hobby, add theiraliasto the title screen, and then distribute the "cracked" product to the network ofwarezBBSesor Internet sites that specialized in distributing unauthorized copies of software. When computer software was still distributed in audio cassettes, audio copying was unreliable, while digital copying was time consuming. Software prices were comparable with audio cassette prices.[4][8]To make digital copying more difficult, many programs used non-standard loading methods (loaders incompatible with standard BASIC loaders, or loaders that used different transfer speed). Unauthorized software copying began to be a problem when floppy disks became the common storage media.[8]The ease of copying depended on the system;Jerry Pournellewrote inBYTEin 1983 that "CP/Mdoesn't lend itself to copy protection" so its users "haven't been too worried" about it, while "Appleusers, though, have always had the problem. So have those who usedTRS-DOS, and I understand thatMS-DOShas copy protection features".[9] Pournelle disliked copy protection[10]and, except for games, refused to review software that used it. He did not believe that it was useful, writing in 1983 that "For every copy protection scheme there's a hacker ready to defeat it. Most involve so-callednibble/nybblecopiers, which try to analyze the original disk and then make a copy".[9]In 1985, he wrote that "dBASE IIIis copy-protected with one of those 'unbreakable' systems, meaning that it took the crackers almost three weeks to break it".[11]IBM'sDon Estridgeagreed: "I guarantee that whatever scheme you come up with will take less time to break than to think of it." While calling piracy "a threat to software development. It's going to dry up the software", he said "It's wrong to copy-protect programs... There ought to be some way to stop [piracy] without creating products that are unusable".[12] Software vendors disliked piracy, but did not attempt to sue the vendors of nibble copiers because copyright law allows users to produce backup copies of software. Unlikemusic piracy, no significant software piracy market existed. A more serious problem was a company or other large organization purchasing a single copy of an application and producing many copies for itself.[13]Philippe KahnofBorlandjustified copy-protectingSidekickbecause, unlike his company's unprotectedTurbo Pascal, Sidekick can be used without accompanying documentation and is for a general audience. Kahn said, according to Pournelle, that "any good hacker can defeat the copy protection in about an hour"; its purpose was to prevent large companies from purchasing one copy and easily distributing it internally. While reiterating his dislike of copy protection, Pournelle wrote "I can see Kahn's point".[14] In 1989,Gilman Louie, head ofSpectrum HoloByte, stated that copy protection added about $0.50 per copy to the cost of production of a game.[15]Other software relied on complexity;Anticin 1988 observed thatWordPerfectfor theAtari ST"is almost unusable without its manual of over 600 pages!".[16](The magazine was mistaken; the ST version was so widely pirated that the company threatened to discontinue it.[17][18]) Copy protection sometimes causes software not to run onclones, such as the Apple II‑compatibleLaser 128,[19]or even the genuineCommodore 64with certain peripherals.[20] To limit reusing activation keys to install the software on multiple machines, it has been attempted to tie the installed software to a specific machine by involving some unique feature of the machine.Serial numberinROMcould not be used because some machines do not have them. Some popular surrogate for a machine serial number were date and time (to the second) of initialization of thehard disk driveorMAC addressof Ethernet cards (although this is programmable on modern cards). With the rise ofvirtualization, however, the practice of locking has to add to these simple hardware parameters to still prevent copying.[21] During the 1980s and 1990s, video games sold onaudio cassetteandfloppy diskswere sometimes protected with an external user-interactive method that demanded the user to have the original package or a part of it, usually the manual. Copy protection was activated not only at installation, but every time the game was executed.[22][23] Several imaginative and creative methods have been employed, in order to be both fun and hard to copy. These include:[24] All of these methods proved to be troublesome and tiring for the players, and as such greatly declined in usage by the mid-1990s, at which point the emergence of CDs as the primary video game medium made copy protection largely redundant, since CD copying technology was not widely available at the time.[22] Somegame developers, such asMarkus Persson,[28]have encouraged consumers and other developers to embrace the reality of unlicensed copying and utilize it positively to generate increased sales and marketing interest. Starting in 1985 with the video release ofThe Cotton Club(BetaandVHSversions only),Macrovisionlicensed to publishers a technology that exploits theautomatic gain controlfeature of VCRs by adding pulses to the vertical blanking sync signal.[29]These pulses may negatively affect picture quality, but succeed in confusing the recording-level circuitry of many consumer VCRs. This technology, which is aided by U.S. legislation mandating the presence of automatic gain-control circuitry in VCRs, is said to "plug the analog hole" and make VCR-to-VCR copies impossible, although an inexpensive circuit is widely available that will defeat the protection by removing the pulses. Macrovision had patented methods of defeating copy prevention,[30]giving it a more straightforward basis to shut down manufacture of any device that descrambles it than often exists in the DRM world. While used for pre‑recorded tapes, the system was not adopted for television broadcasts;Michael J. FuchsofHBOsaid in 1985 that Macrovision was "not good technology" because it reduced picture quality and consumers could easily bypass it, whilePeter CherninofShowtimesaid "we want to accommodate our subscribers and we know they like to tape our movies".[31] Over time, software publishers (especially in the case ofvideo games) became creative about crippling the software in case it was duplicated. These games would initially show that the copy was successful, but eventually render themselves unplayable via subtle methods. Many games use the "code checksumming" technique to prevent alteration of code to bypass other copy protection. Important constants for the game – such as the accuracy of the player's firing, the speed of their movement, etc. – are not included in the game but calculated from the numbers making up themachine codeof other parts of the game. If the code is changed, the calculation yields a result which no longer matches the original design of the game and the game plays improperly. Copying commercial games, such as this one, is a criminal offense and copyright infringement. Copying and re-supplying games such as this one can lead to a term of imprisonment.Think of a pirated game as stolen property.This game is protected by the FADE system. You can play with a pirated game- but not for long. The quality of a pirated game will degrade over time. Purchase only genuine software at legitimate stores. The usage of copy protection payloads which lower playability of a game without making it clear that this is a result of copy protection is now generally considered unwise, due to the potential for it to result in unaware players with unlicensed copies spreading word-of-mouth that a game is of low quality. The authors ofFADEexplicitly acknowledged this as a reason for including the explicit warning message. Anti-piracy measures are efforts to fight againstcopyright infringement,counterfeiting, and other violations ofintellectual propertylaws. It includes, but is by no means limited to, the combined efforts of corporate associations (such as theRIAAandMPA), law enforcement agencies (such as theFBIandInterpol), and various international governments[clarification needed]to combat copyright infringement relating to various types of creative works, such as software, music and films. These measures often come in the form of copy protection measures such asDRM, or measures implemented through acontent protection network, such as Distil Networks or Incapsula.Richard Stallmanand theGNU Projecthave criticized the use of the word "piracy" in these situations, saying that publishers use the word to refer to "copying they don't approve of" and that "they [publishers] imply that it is ethically equivalent to attacking ships on the high seas, kidnapping and murdering the people on them".[42]Certain forms of anti-piracy (such as DRM) are considered by consumers to control the use of the products content aftersale. In the caseMPAA v. Hotfile, JudgeKathleen M. Williamsgranted a motion to deny the prosecution the usage of words she views as "pejorative". This list included the word "piracy", the use of which, the motion by the defense stated, would serve no purpose but to misguide and inflame the jury. The plaintiff argued the common use of the terms when referring to copyright infringement should invalidate the motion, but the Judge did not concur.[43] Today, copyright infringement is often facilitated by the use offile sharing. In fact, infringement accounts for 23.8% of all internet traffic in 2013.[44]In an effort to cut down on this, both large and small films and music corporations have issuedDMCAtakedown notices, filed lawsuits, and pressed criminal prosecution of those who host these file sharing services.[45][46][47][48] TheEURion constellationis used by many countries to prevent color photocopiers from producingcounterfeit currency. The Counterfeit Deterrence System is used to prevent counterfeit bills from being produced by image editing software.[49]Similar technology has been proposed[50]to prevent3D printing of firearms, for reasons ofgun controlrather than copyright.	https://en.wikipedia.org/wiki/Copy_protection
Acybersecurity regulationcomprises directives that safeguardinformation technologyandcomputer systemswith the purpose of forcing companies and organizations to protect their systems and information fromcyberattackslikeviruses,worms,Trojan horses,phishing,denial of service (DOS) attacks,unauthorized access (stealing intellectual property or confidential information)andcontrol system attacks.[1]While cybersecurity regulations aim to minimize cyber risks and enhance protection, the uncertainty arising from frequent changes or new regulations can significantly impact organizational response strategies.[1] There are numerous measures available to preventcyberattacks.Cybersecuritymeasures includefirewalls,anti-virus software,intrusion detectionandpreventionsystems,encryption, and loginpasswords.[2]There have been attempts to improve cybersecurity through regulation and collaborative efforts between thegovernmentand the private sector to encourage voluntary improvements to cybersecurity.[1][2][3]Industry regulators, includingbanking regulators, have taken notice of the risk from cybersecurity and have either begun or planned to begin to include cybersecurity as an aspect of regulatory examinations.[2] Recent research suggests there is also a lack of cyber-security regulation and enforcement in maritime businesses, including the digital connectivity between ships and ports.[4] In 2011 theUS DoDreleased a guidance called theDepartment of Defense Strategy for Operating in Cyberspacewhich articulated five goals: to treat cyberspace as an operational domain, to employ new defensive concepts to protect DoD networks and systems, to partner with other agencies and the private sector in pursuit of a "whole-of-government cybersecurity Strategy", to work with international allies in support of collective cybersecurity and to support the development of a cyber workforce capable of rapid technological innovation.[3]A March 2011GAOreport "identified protecting the federal government's information systems and the nation's cyber critical infrastructure as a governmentwide high-risk area" noting that federalinformation securityhad been designated a high-risk area since 1997. As of 2003 systems protecting critical infrastructure, called cyber critical infrastructure protection of cyber CIP has also been included.[5] In November 2013, the DoD put forward the new cybersecurity rule (78 Fed. Reg. 69373), which imposed certain requirements on contractors: compliance with certainNISTIT standards, mandatory reporting of cybersecurity incidents to the DoD, and a "flow-down" clause that applies the same requirements to subcontractors.[6] A June 2013 Congressional report found there were over 50 statutes relevant to cybersecurity compliance. TheFederal Information Security Management Act of 2002(FISMA) is one of the key statutes governing federal cybersecurity regulations.[6] There are few federal cybersecurity regulations and the ones that exist focus on specific industries. The three main cybersecurity regulations are the 1996Health Insurance Portability and Accountability Act(HIPAA), the 1999Gramm-Leach-Bliley Act, and the 2002Homeland Security Act, which included theFederal Information Security Management Act(FISMA). The three regulations mandate that healthcare organizations, financial institutions, and federal agencies should protect their systems and information.[3]For example, FISMA, which applies to every government agency, "requires the development and implementation of mandatory policies, principles, standards, and guidelines on information security." However, the regulations do not address numerous computer-related industries, such asInternet Service Providers(ISPs) and software companies.[4]Furthermore, the regulations do not specify what cybersecurity measures must be implemented and require only a "reasonable" level of security. The vague language of these regulations leaves much room for interpretation.Bruce Schneier, the founder of Cupertino's Counterpane Internet Security, argues that companies will not make sufficient investments in cybersecurity unless the government forces them to do so.[5]He also states that successful cyberattacks on government systems still occur despite government efforts.[6] It has been suggested that theData Quality Actalready provides theOffice of Management and Budgetthe statutory authority to implementcritical infrastructure protectionregulations by theAdministrative Procedure Actrulemaking process. The idea has not been fully vetted and would require additional legal analysis before arulemakingcould begin.[7] State governments have attempted to improve cybersecurity by increasing public visibility of firms with weak security. In 2003,Californiapassed the Notice of Security Breach Act, which requires that any company that maintains personal information of California citizens and has a security breach must disclose the details of the event. Personal information includes name,social security number, driver's license number,credit card numberor financial information.[7]Several other states have followed California's example and passed similar security breach notification regulations.[8]Such security breach notification regulations punish firms for their cybersecurity failures while giving them the freedom to choose how to secure their systems. Also, the regulation creates an incentive for companies to voluntarily invest in cybersecurity to avoid the potential loss of reputation and the resulting economic loss that can come from a successful cyber attack.[8] In 2004, theCalifornia State Legislaturepassed California Assembly Bill 1950, which also applies to businesses that own or maintain personal information for California residents. The regulation dictates for businesses to maintain a reasonable level of security and that they required security practices also extend to business partners.[9]The regulation is an improvement on the federal standard because it expands the number of firms required to maintain an acceptable standard of cybersecurity. However, like the federal legislation, it requires a "reasonable" level of cybersecurity, which leaves much room for interpretation until case law is established.[10] TheUS Congresshas proposed numerous bills that expand upon cybersecurity regulation. TheConsumer Data Security and Notification Actamends theGramm-Leach-Bliley Actto require disclosure of security breaches by financial institutions. Congressmen have also proposed "expanding Gramm-Leach-Bliley to all industries that touch consumer financial information, including any firm that accepts payment by a credit card."[11]Congress has proposed cybersecurity regulations similar to California's Notice of Security Breach Act for companies that maintain personal information. The Information Protection and Security Act requires that data brokers "ensure data accuracy and confidentiality, authenticate and track users, detect and prevent unauthorized activity, and mitigate potential harm to individuals."[12] In addition to requiring companies to improve cybersecurity, Congress is also considering bills that criminalize cyberattacks. The Securely Protect Yourself Against Cyber Trespass Act (SPY ACT) was a bill of this type. It focused on phishing andspywarebill and was passed on May 23, 2005, in theUS House of Representativesbut died in theUS Senate.[8]The bill "makes unlawful the unauthorized usage of a computer to take control of it, modify its setting, collect or induce the owner to disclosepersonally identifiable information, install unsolicited software, and tamper with security, anti-spyware, oranti-virus software."[13] On May 12, 2011, US presidentBarack Obamaproposed a package ofcybersecurity legislative reformsto improve the security of US persons, the federal government, and critical infrastructure. A year of public debate and Congress hearings followed, resulting in the House of Representative passing aninformation sharing billand the Senate developing acompromise billseeking to balance national security, privacy, and business interests. In July 2012, the Cybersecurity Act of 2012 was proposed by SenatorsJoseph LiebermanandSusan Collins.[14]The bill would have required creating voluntary "best practice standards" for protection of key infrastructure from cyber attacks, which businesses would be encouraged to adopt through incentives such as liability protection.[15]The bill was put to a vote in the Senate but failed to pass.[16]Obama had voiced his support for the Act in aWall Street Journalop-ed[17], and it also received support from officials in the military and national security includingJohn O. Brennan, the chief counterterrorism adviser to the White House.[18][19]According toThe Washington Post, experts said that the failure to pass the act may leave the United States "vulnerable to widespread hacking or a serious cyberattack."[20]The act was opposed by Republican senators likeJohn McCainwho was concerned that the act would introduce regulations that would not be effective and could be a "burden" for businesses.[21]After the Senate vote, Republican SenatorKay Bailey Hutchisonstated that the opposition to the bill was not a partisan issue but it not take the right approach to cybersecurity.[22]The senate vote was not strictly along partisan lines, as six Democrats voted against it, and five Republicans voted for it.[23]Critics of the bill included theUS Chamber of Commerce,[24]advocacy groups like theAmerican Civil Liberties Unionand theElectronic Frontier Foundation,[25]cybersecurity expert Jody Westby, andThe Heritage Foundation, both of whom argued that although the government must act on cybersecurity, the bill was flawed in its approach and represented "too intrusive a federal role."[26] In February 2013, Obama proposed the Executive Order Improving Critical Infrastructure Cybersecurity. It represents the latest iteration of policy but is not considered to be law as it has not been addressed by Congress yet. It seeks to improve existing public-private partnerships by enhancing timeliness of information flow between DHS and critical infrastructure companies. It directs federal agencies to share cyber threat intelligence warnings to any private sector entity identified as a target. It also tasks DHS with improving the process to expedite security clearance processes for applicable public and private sector entities to enable the federal government to share this information at the appropriate sensitive and classified levels. It directs the development of a framework to reduce cyber risks, incorporating current industry best practices and voluntary standards. Lastly, it tasks the federal agencies involved with incorporating privacy and civil liberties protections in line with Fair Information Practice Principles.[9] In January 2015, Obama announced a new cybersecurity legislative proposal. The proposal was made in an effort to prepare the US from the expanding number of cyber crimes. In the proposal, Obama outlined three main efforts to work towards a more secure cyberspace for the US. The first main effort emphasized the importance of enabling cybersecurity information sharing. By enabling that, the proposal encouraged information sharing between the government and the private sector. That would allow the government to know what main cyber threats private firms are facing and would then allow the government to provide liability protection to those firms that shared their information. Furthermore, that would give the government a better idea of what the US needs to be protected against. Another main effort that was emphasized in this proposal was to modernize the law enforcement authorities to make them more equipped to properly deal with cyber crimes by giving them the tools they need in order to do so. It would also update classifications of cyber crimes and consequences. One way this would be done would be by making it a crime for overseas selling of financial information. Another goal of the effort is to place cyber crimes prosecutable. The last major effort of the legislative proposal was to require businesses to report data breaching to consumers if their personal information had been sacrificed. By requiring companies to do so, consumers are aware of when they are in danger of identity theft.[10] In February 2016, Obama developed a Cybersecurity National Security Action Plan (CNAP). The plan was made to create long-term actions and strategies in an effort to protect the US against cyber threats. The focus of the plan was to inform the public about the growing threat of cyber crimes, improve cybersecurity protections, protects personal information of Americans, and to inform Americans on how to control digital security. One of the highlights of this plan include creating a "Commission on Enhancing National Cybersecurity." The goal of this is to create a Commission that consists of a diverse group of thinkers with perspectives that can contribute to make recommendations on how to create a stronger cybersecurity for the public and private sector. The second highlight of the plan is to change Government IT. The new Government IT will make it so that a more secure IT can be put in place. The third highlight of the plan is to give Americans knowledge on how they can secure their online accounts and avoid theft of their personal information through multi-factor authentication. The fourth highlight of the plan is to invest 35% more money that was invested in 2016 into cybersecurity.[11] In addition to regulation, the federal government has tried to improve cybersecurity by allocating more resources to research and collaborating with the private sector to write standards. In 2003, the President'sNational Strategy to Secure Cyberspacemade theDepartment of Homeland Security(DHS) responsible for security recommendations and researching national solutions. The plan calls for cooperative efforts between government and industry "to create an emergency response system to cyber-attacks and to reduce the nation's vulnerability to such threats "[27]In 2004, the US Congress allocated $4.7 billion toward cybersecurity and achieving many of the goals stated in the President's National Strategy to Secure Cyberspace.[28]Some industry security experts state that the President's National Strategy to Secure Cyberspace is a good first step but is insufficient.[29]Bruce Schneier stated, "The National Strategy to Secure Cyberspace hasn't secured anything yet."[30]However, the President's National Strategy clearly states that the purpose is to provide a framework for the owners of computer systems to improve their security rather than the government taking over and solving the problem.[31]However, companies that participate in the collaborative efforts outlined in the strategy are not required to adopt the discovered security solutions. In the United States, theUS Congressis trying to make information more transparent after the Cyber Security Act of 2012, which would have created voluntary standards for protecting vital infrastructure, failed to pass through the Senate.[12]In February 2013, theWhite Houseissued an executive order, titled "Improving Critical Infrastructure Cybersecurity," which allows theexecutive branchto share information about threats with more companies and individuals.[12][13]In April 2013, the House of Representatives passed theCyber Intelligence Sharing and Protection Act(CISPA), which calls for protecting against lawsuits aimed at companies that disclose breach information.[12]TheObama administrationsaid that it might veto the bill.[12] In the light of the hacking of the website of theIndian Space Agency's commercial arm in 2015, Antrix Corporation and government's Digital India programme, a cyberlaw expert and advocate at theSupreme Court of India,Pavan Duggal, stated that "a dedicated cyber security legislation as a key requirement for India. It is not sufficient to merely put cyber security as a part of the IT Act. We have to see cyber security not only from the sectoral perspective, but also from the national perspective."[14] Cybersecurity standards have been of great prominence in today's technology driven businesses. To maximize their profits, corporations leverage technology by running most of their operations by the internet. Since there are a large number of risks that entail internetwork operations, such operations must be protected by comprehensive and extensive regulations. Existing cybersecurity regulations all cover different aspects of business operations and often vary by region or country in which a business operates. Because of the differences in a country's society, infrastructure, and values, one overarching cyber security standard is not optimal for decreasing risks. While US standards provide a basis for operations, theEuropean Unionhas created a more tailored regulation for businesses operating specifically within the EU. Also, in light ofBrexit, it is important to consider how the UK has chosen to adhere to such security regulations. Three major regulations within the EU include the ENISA, the NIS Directive and the EU GDPR. They are part of theDigital Single Marketstrategy. Regarding standards, the Cybersecurity Act / ENISA Regulation does not refer directly to standards. Nevertheless, ENISA recognises on its website that "EU’s cybersecurity strategy underscores support for greater standardisation via the European standardisation organisations (CEN, CENELEC and ETSI) as well as ISO.[15]" ISO/IEC Standards, as well as European Standards from CEN, CENELEC and ETSI can be used on a voluntary way to support the requirements in the EU legislation. An updated list of ISO/IEC and CEN/CENELEC standards on the topic of Cybersecurity can be followed up via the free and publicly available information website Genorma.com.[16] TheEuropean Union Agency for Cybersecurity(ENISA) is a governing agency that was originally set up by the Regulation (EC) No 460/2004 of the European Parliament and of the Council of 10 March 2004 for the Purpose of Raising Network and Information Security (NIS) for all internetwork operations in the EU. ENISA currently runs under Regulation (EU) No 526/2013,[17]which has replaced the original regulation in 2013. ENISA works actively with all member states of the EU to provide a range of services. The focus of their operations are on three factors: ENISA is made up of a management board that relies on the support of the executive director and the Permanent Stakeholders Group. Most operations, however, are run by the heads of various departments.[19] ENISA has released various publications that cover all major issues on cybersecurity. ENISA's past and current initiatives include the EU Cloud Strategy, Open Standards in Information Communications Technology, a Cyber Security Strategy of the EU and a Cyber Security Coordination Group. ENISA also works in collaboration with existing international standard organizations like theISOand theITU.[20] On July 6, 2016, the European Parliament set into policy theDirective on Security of Network and Information Systems(theNIS Directive).[21] The directive went into effect in August 2016, and all member states of the European Union were given 21 months to incorporate the directive's regulations into their own national laws.[22]The aim of the NIS Directive is to create an overall higher level of cybersecurity in the EU. The directive significantly affects digital service providers (DSPs) and operators of essential services (OESs). Operators of essential services include any organizations whose operations would be greatly affected in the case of a security breach if they engage in critical societal or economic activities. Both DSPs and OES are now held accountable for reporting major security incidents to Computer Security Incident Response Teams (CSIRT).[23]While DSPs are not held to as stringent regulations as operators of essential services, DSPs that are not set up in the EU but still operate in the EU still face regulations. Even if DSPs and OES outsource the maintenance of their information systems to third parties, the NIS Directive still holds them accountable for any security incidents.[24] The member states of the EU are required to create a NIS directive strategy, which includes the CSIRTs, in addition to National Competent Authorities (NCAs) and Single Points of Contact (SPOCs). Such resources are given the responsibility of handling cybersecurity breaches in a way that minimizes impact. In addition, all member states of the EU are encouraged to share cyber security information.[25] Security requirements include technical measures that manage the risks of cybersecurity breaches in a preventative manner. Both DSP and OES must provide information that allows for an in depth assessment of their information systems and security policies.[26]All significant incidents must be notified to the CSIRTs. Significant cybersecurity incidents are determined by the number of users affected by the security breach as well as the longevity of the incident and the geographical reach of the incident.[26]A NIS 2 is in the making.[27] Only 23 Member States have fully implemented the measures contained with the NIS Directive. Infringement proceedings against them to enforce the Directive have not taken place, and they are not expected to take place in the near future.[28]This failed implementation has led to the fragmentation of cybersecurity capabilities across the EU, with differing standards, incident reporting requirements and enforcement requirements being implemented in different Member States. The EU CybersecurityActestablishes an EU-wide cybersecurity certification framework for digital products, services and processes. It complements the NIS Directive.ENISAwill have a key role in setting up and maintaining the European cybersecurity certification framework.[29] The EUGeneral Data Protection Regulation(GDPR) was set into place on 14 April 2016, but the current date of enforcement is set to be on 25 May 2018.[30]The GDPR aims to bring a single standard for data protection among all member states in the EU. Changes include the redefining of geographical borders. It applies to entities that operate in the EU or deal with the data of any resident of the EU. Regardless of where the data is processed, if an EU citizen's data is being processed, the entity is now subject to the GDPR.[31] Fines are also much more stringent under the GDPR and can total €20 million or 4% of an entity's annual turnover, whichever is higher.[31]In addition, like in previous regulations, all data breaches that effect the rights and freedoms of individuals residing in the EU must be disclosed within 72 hours. The overarching board, the EU Data Protection Board, EDP, is in charge of all oversight set by the GDPR. Consent plays a major role in the GDPR. Companies that hold data in regards to EU citizens must now also offer to them the right to back out of sharing data just as easily as when they consented to sharing data.[32] In addition, citizens can also restrict processing of the data stored on them and can choose to allow companies to store their data but not process it, which creates a clear differentiation. Unlike previous regulations, the GDPR also restricts the transfer of a citizen's data outside of the EU or to a third party without a citizen's prior consent.[32] On the 16 January 2023, the EU Parliament and Council adopted the 2022/2555 of the European Parliament and of the Council of 14 December 2022 on measures for a high common level of cybersecurity across the Union, amending Regulation (EU) No 910/2014 and Directive (EU) 2018/1972, and repealing Directive (EU) 2016/1148 (NIS Directive). This new Directive aims to extend the scope of obligations on entities required to take measures to increase their cybersecurity capabilities. The Directive also aims to harmonise the EU approach to incident notifications, security requirements, supervisory measures and information sharing.[33]TheNational Cyber Security Bill 2024will transpose NIS2 into Irish law once enacted. The National Cyber Security Bill 2024 represents a proactive step towards safeguarding Ireland's critical infrastructure and enhancing its overall cybersecurity resilience in line with European standards, as stipulated in NIS2.[34] DORA creates a regulatory framework on digital operational resilience whereby all firms need to make sure they can withstand, respond to and recover from all types of ICT-related disruptions and threats. These requirements are homogenous across all EU member states. The regulation will apply from 17 January 2025 for relevant financial entities and ICT third-party service providers.[35] The Cyber Resilience Act (CRA) is aregulationproposed on 15 September 2022 by the European Commission which outlines common cybersecurity standards for hardware and software products in the EU.[36][37][38] The Criminal Justice (Offences Relating to Information Systems) Act 2017 was introduced in May 2017 to consolidate laws on computer crime.[39][40] While experts agree that cybersecurity improvements are necessary, there is disagreement about whether the solution is more government regulation or more private-sector innovation. Many government officials and cybersecurity experts believe that the private sector has failed to solve the cybersecurity problem and that regulation is needed.Richard Clarkestates that "industry only responds when you threaten regulation. If industry does not respond [to the threat], you have to follow through."[32]He believes that software companies must be forced to produce more secure programs.[33]Bruce Schneieralso supports regulation that encourages software companies to write more secure code through economic incentives.[34]US RepresentativeRick Boucher(D–VA) proposes improving cybersecurity by making software companies liable for security flaws in their code.[35]In addition, to improving software security, Clarke believes that certain industries, such as utilities and ISPs, require regulation.[36] On the other hand, many private-sector executives and lobbyists believe that more regulation will restrict their ability to improve cybersecurity. Harris Miller, alobbyistand president of theInformation Technology Association of America, believes that regulation inhibits innovation.[37]Rick White, former corporate attorney and president andCEOof the lobby group TechNet, also opposes more regulation. He states that "the private-sector must continue to be able to innovate and adapt in response to new attack methods in cyber space, and toward that end, we commend President Bush and the Congress for exercising regulatory restraint."[38] Another reason many private-sector executives oppose regulation is that it is costly and involves government oversight in private enterprise. Firms are just as concerned about regulation reducing profits as they are about regulation limiting their flexibility to solve the cybersecurity problem efficiently. Specifically around the CRA, concern is expressed over the breadth of impact by prominent free andOpen sourcesoftware organizations:Eclipse Foundation,Internet Society, andPython Software Foundation. These organizations raise several consequences which are unstated by the regulation, that they conclude fundamentally damage the Open source movement. They offer changes that would allow Open source to be used in the EU without being regulated in the same manner as would be on commercial software developers.[41][42][43][44]	https://en.wikipedia.org/wiki/Cyber-security_regulation
Data erasure(sometimes referred to asdata clearing,data wiping, ordata destruction) is a software-based method ofdata sanitizationthat aims to completely destroy allelectronic data residingon ahard disk driveor otherdigital mediaby overwriting data onto all sectors of the device in anirreversible process. By overwriting the data on the storage device, the data is rendered irrecoverable. Ideally, software designed for data erasure should: Permanent data erasure goes beyond basicfile deletioncommands, which only remove direct pointers to the datadisk sectorsand make thedata recoverypossible with common software tools. Unlikedegaussingand physical destruction, which render the storage media unusable, data erasure removes all information while leaving the disk operable. Newflash memory-based media implementations, such assolid-state drivesorUSB flash drives, can cause data erasure techniques to fail allowingremnant datato be recoverable.[1] Software-based overwriting uses a software application to write a stream of zeros, ones or meaningless pseudorandom data onto all sectors of a hard disk drive. There are key differentiators between data erasure and other overwriting methods, which can leave data intact and raise the risk ofdata breach,identity theftor failure to achieve regulatory compliance. Many data eradication programs also providemultiple overwritesso that they support recognized government and industry standards, though a single-pass overwrite is widely considered to be sufficient for modern hard disk drives. Good software should provide verification of data removal, which is necessary for meeting certain standards. To protect the data on lost or stolen media, some data erasure applications remotely destroy the data if the password is incorrectly entered. Data erasure tools can also target specific data on a disk for routine erasure, providing ahackingprotection method that is less time-consuming than software encryption.Hardware/firmwareencryption built into the drive itself or integrated controllers is a popular solution with no degradation in performance at all. When encryption is in place, data erasure acts as a complement tocrypto-shredding, or the practice of 'deleting' data by (only) deleting or overwriting the encryption keys.[2] Presently, dedicated hardware/firmware encryption solutions can perform a 256-bit fullAES encryptionfaster than the drive electronics can write the data. Drives with this capability are known as self-encrypting drives (SEDs); they are present on most modern enterprise-level laptops and are increasingly used in the enterprise to protect the data. Changing the encryption key renders inaccessible all data stored on a SED, which is an easy and very fast method for achieving a 100% data erasure. Theft of an SED results in a physical asset loss, but the stored data is inaccessible without the decryption key that is not stored on a SED, assuming there are no effective attacks against AES or its implementation in the drive hardware.[citation needed] Information technology assets commonly hold large volumes of confidential data.Social security numbers, credit card numbers, bank details, medical history and classified information are often stored on computer hard drives orservers. These can inadvertently or intentionally make their way onto other media such as printers,USB,flash,Zip,Jaz, andREVdrives. Increased storage of sensitive data, combined with rapid technological change and the shorter lifespan of IT assets, has driven the need for permanent data erasure of electronic devices as they are retired or refurbished. Also, compromised networks andlaptop theftand loss, as well as that of other portable media, are increasingly common sources of data breaches. If data erasure does not occur when a disk is retired or lost, an organization or user faces a possibility that the data will be stolen and compromised, leading to identity theft, loss of corporate reputation, threats to regulatory compliance and financial impacts. Companies spend large amounts of money to make sure their data is erased when they discard disks.[3][dubious–discuss]High-profile incidents of data theft include: Strict industry standards and government regulations are in place that force organizations to mitigate the risk of unauthorized exposure of confidential corporate and government data. Regulations in theUnited StatesincludeHIPAA(Health Insurance Portability and Accountability Act);FACTA(The Fair and Accurate Credit Transactions Act of 2003); GLB (Gramm-Leach Bliley);Sarbanes-Oxley Act(SOx); and Payment Card Industry Data Security Standards (PCI DSS) and theData Protection Actin theUnited Kingdom. Failure to comply can result in fines and damage to company reputation, as well as civil and criminal liability.[citation needed] Data erasure offers an alternative to physical destruction and degaussing for secure removal of all the disk data. Physical destruction and degaussing destroy the digital media, requiring disposal and contributing toelectronic wastewhile negatively impacting thecarbon footprintof individuals and companies.[10]Hard drives are nearly 100% recyclable and can be collected at no charge from a variety of hard drive recyclers after they have been sanitized.[11] Data erasure may not work completely on flash based media, such asSolid State DrivesandUSB Flash Drives, as these devices can store remnant data which is inaccessible to the erasure technique, and data can be retrieved from the individual flash memory chips inside the device.[1]Data erasure through overwriting only works on hard drives that are functioning and writing to all sectors.Bad sectorscannot usually be overwritten, but may contain recoverable information. Bad sectors, however, may beinvisibleto the host system and thus to the erasing software.Disk encryptionbefore use prevents this problem. Software-driven data erasure could also be compromised by malicious code.[12] Software-based data erasure uses a disk accessible application to write a combination of ones, zeroes and any otheralpha numeric characteralso known as the "mask" onto each hard disk drive sector. The level ofsecuritywhen using software data destruction tools is increased dramatically by pre-testing hard drives for sector abnormalities and ensuring that the drive is 100% in working order. The number of wipes has become obsolete with the more recent inclusion of a "verify pass" which scans all sectors of the disk and checks against what character should be there, i.e., one pass of AA has to fill every writable sector of the hard disk. This makes any more than one pass an unnecessary and certainly a more damaging act, especially in the case of large multi-terabyte drives. While there are many overwriting programs, only those capable of complete data erasure offer full security by destroying the data on all areas of a hard drive. Disk overwriting programs that cannot access the entire hard drive, including hidden/locked areas like thehost protected area(HPA),device configuration overlay(DCO), and remapped sectors, perform an incomplete erasure, leaving some of the data intact. By accessing the entire hard drive, data erasure eliminates the risk ofdata remanence. Data erasure can also bypass theOperating System (OS). Overwriting programs that operate through the OS will not always perform a complete erasure because they cannot modify the contents of the hard drive that are actively in use by that OS. Because of this, many data erasure programs are provided in a bootable format, where you run off alive CDthat has all of the necessary software to erase the disk.[citation needed] Data erasure can be deployed over a network to target multiplePCsrather than having to erase each one sequentially. In contrast withDOS-based overwriting programs that may not detect all network hardware,Linux-based data erasure software supports high-end server andstorage area network(SAN) environments with hardware support forSerial ATA,Serial Attached SCSI(SAS) andFibre Channeldisks and remapped sectors. It operates directly with sector sizes such as 520, 524, and 528, removing the need to first reformat back to 512 sector size.WinPEhas now overtaken Linux as the environment of choice since drivers can be added with little effort. This also helps with data destruction of tablets and other handheld devices that require pure UEFI environments without hardware NIC's installed and/or are lacking UEFI network stack support. Many government and industry standards exist for software-based overwriting that removes the data. A key factor in meeting these standards is the number of times the data is overwritten. Also, some standards require a method to verify that all the data have been removed from the entire hard drive and to view the overwrite pattern. Complete data erasure should account for hidden areas, typically DCO, HPA and remapped sectors. The 1995 edition of theNational Industrial Security ProgramOperating Manual (DoD 5220.22-M) permitted the use of overwriting techniques to sanitize some types of media by writing all addressable locations with a character, its complement, and then a random character. This provision was removed in a 2001 change to the manual and was never permitted for Top Secret media, but it is still listed as a technique by many providers of the data erasure software.[13] Data erasure software should provide the user with avalidationcertificate indicating that the overwriting procedure was completed properly. Data erasure software should[citation needed]also comply with requirements to erase hidden areas, provide a defects log list and list bad sectors that could not be overwritten. Alternative:Random character (byte), its complement, another random character. Head stepping direction should alternate between tests. Read caching disabled. Data can sometimes be recovered from a broken hard drive. However, if theplatterson a hard drive are damaged, such as by drilling a hole through the drive (and the platters inside), then the data can only theoretically be recovered by bit-by-bit analysis of each platter with advanced forensic technology. Data onfloppy diskscan sometimes be recovered by forensic analysis even after the disks have been overwritten once with zeros (or random zeros and ones).[27] This is not the case with modern hard drives: Even the possibility of recovering floppy disk data after overwrite is disputed. Gutmann's famous article cites a non-existent source and sources that do not actually demonstrate recovery, only partially-successful observations. Gutmann's article also contains many assumptions that indicate his insufficient understanding of how hard drives work, especially the data processing and encoding process.[31]The definition of "random" is also quite different from the usual one used: Gutmann expects the use of pseudorandom data with sequences known to the recovering side, not an unpredictable one such as acryptographically secure pseudorandom number generator.[32] E-waste presents a potentialsecuritythreat to individuals and exporting countries.Hard drivesthat are not properly erased before the computer is disposed of can be reopened, exposingsensitive information.Credit cardnumbers, private financial data, account information and records of online transactions can be accessed by most willing individuals. Organized criminals inGhanacommonly search the drives for information to use in localscams.[33] Government contracts have been discovered on hard drives found inAgbogbloshie.[citation needed]	https://en.wikipedia.org/wiki/Data_erasure
Incomputing,data recoveryis a process of retrieving deleted, inaccessible, lost, corrupted, damaged, overwritten or formatted data fromsecondary storage,removable mediaorfiles, when the data stored in them cannot be accessed in a usual way.[1]The data is most often salvaged from storage media such as internal or externalhard disk drives(HDDs),solid-state drives(SSDs),USB flash drives,magnetic tapes,CDs,DVDs,RAIDsubsystems, and otherelectronic devices. Recovery may be required due to physical damage to the storage devices or logical damage to thefile systemthat prevents it from beingmountedby the hostoperating system(OS).[1] Logical failures occur when the hard drive devices are functional but the user or automated-OS cannot retrieve or access data stored on them. Logical failures can occur due to corruption of the engineering chip, lost partitions, firmware failure, or failures during formatting/re-installation.[2][3] Data recovery can be a very simple or technical challenge. This is why there are specific software companies specialized in this field.[4] The most common data recovery scenarios involve an operating system failure, malfunction of a storage device, logical failure of storage devices, accidental damage or deletion, etc. (typically, on a single-drive, single-partition, single-OS system), in which case the ultimate goal is simply to copy all important files from the damaged media to another new drive. This can be accomplished using aLive CD, or DVD by booting directly from aROMor a USB drive instead of the corrupted drive in question. Many Live CDs or DVDs provide a means to mount the system drive and backup drives or removable media, and to move the files from the system drive to the backup media with afile manageroroptical disc authoringsoftware. Such cases can often be mitigated bydisk partitioningand consistently storing valuable data files (or copies of them) on a different partition from the replaceable OS system files. Another scenario involves a drive-level failure, such as a compromisedfile systemor drive partition, or ahard disk drive failure. In any of these cases, the data is not easily read from the media devices. Depending on the situation, solutions involve repairing the logical file system, partition table, ormaster boot record, or updating thefirmwareor drive recovery techniques ranging from software-based recovery of corrupted data, to hardware- and software-based recovery of damaged service areas (also known as the hard disk drive's "firmware"), to hardware replacement on a physically damaged drive which allows for the extraction of data to a new drive. If a drive recovery is necessary, the drive itself has typically failed permanently, and the focus is rather on a one-time recovery, salvaging whatever data can be read. In a third scenario, files have been accidentally "deleted" from a storage medium by the users. Typically, the contents of deleted files are not removed immediately from the physical drive; instead, references to them in the directory structure are removed, and thereafter space the deleted data occupy is made available for later data overwriting. In the mind ofend users, deleted files cannot be discoverable through a standard file manager, but the deleted data still technically exists on the physical drive. In the meantime, the original file contents remain, often several disconnectedfragments, and may be recoverable if not overwritten by other data files. The term "data recovery" is also used in the context offorensicapplications orespionage, where data which have beenencrypted, hidden, or deleted, rather than damaged, are recovered. Sometimes data present in the computer gets encrypted or hidden due to reasons like virus attacks which can only be recovered by some computer forensic experts. A wide variety of failures can cause physical damage to storage media, which may result from human errors and natural disasters.CD-ROMscan have their metallic substrate or dye layer scratched off; hard disks can suffer from a multitude of mechanical failures, such ashead crashes, PCB failure, and failed motors;tapescan simply break. Physical damage to a hard drive, even in cases where a head crash has occurred, does not necessarily mean permanent data loss. However, in extreme cases, such as prolonged exposure tomoistureandcorrosion—like the lostBitcoin hard drive of James Howells, buried in the Newport landfillfor over a decade — recovery is usually impossible. In rare cases, forensic techniques likeMagnetic Force Microscopy(MFM) have been explored to detect residual magnetic traces when data holds exceptional value.[5]Other techniques employed by many professional data recovery companies can typically salvage most, if not all, of the data that had been lost when the failure occurred. Of course, there are exceptions to this, such as cases where severe damage to the hard driveplattersmay have occurred. However, if the hard drive can be repaired and a full image or clone created, then the logical file structure can be rebuilt in most instances. Most physical damage cannot be repaired by end users. For example, opening a hard disk drive in a normal environment can allow airborne dust to settle on the platter and become caught between the platter and theread/write head. During normal operation, read/write heads float 3 to 6nanometersabove the platter surface, and the average dust particles found in a normal environment are typically around 30,000nanometers in diameter.[6]When these dust particles get caught between the read/write heads and the platter, they can cause new head crashes that further damage the platter and thus compromise the recovery process. Furthermore, end users generally do not have the hardware or technical expertise required to make these repairs. Consequently, data recovery companies are often employed to salvage important data with the more reputable ones usingclass 100dust- and static-freecleanrooms.[7] Recovering data from physically damaged hardware can involve multiple techniques. Some damage can be repaired by replacing parts in the hard disk. This alone may make the disk usable, but there may still be logical damage. A specialized disk-imaging procedure is used to recover every readable bit from the surface. Once this image is acquired and saved on a reliable medium, the image can be safely analyzed for logical damage and will possibly allow much of the original file system to be reconstructed. A common misconception is that a damagedprinted circuit board(PCB) may be simply replaced during recovery procedures by an identical PCB from a healthy drive. While this may work in rare circumstances on hard disk drives manufactured before 2003, it will not work on newer drives. Electronics boards of modern drives usually contain drive-specificadaptation data(generally a map of bad sectors and tuning parameters) and other information required to properly access data on the drive. Replacement boards often need this information to effectively recover all of the data. The replacement board may need to be reprogrammed. Some manufacturers (Seagate, for example) store this information on a serialEEPROMchip, which can be removed and transferred to the replacement board.[8][9] Each hard disk drive has what is called asystem areaorservice area; this portion of the drive, which is not directly accessible to the end user, usually contains drive's firmware and adaptive data that helps the drive operate within normal parameters.[10]One function of the system area is to log defective sectors within the drive; essentially telling the drive where it can and cannot write data. The sector lists are also stored on various chips attached to the PCB, and they are unique to each hard disk drive. If the data on the PCB do not match what is stored on the platter, then the drive will not calibrate properly.[11]In most cases the drive heads will click because they are unable to find the data matching what is stored on the PCB. The term "logical damage" refers to situations in which the error is not a problem in the hardware and requires software-level solutions. In some cases, data on a hard disk drive can be unreadable due to damage to thepartition tableorfile system, or to (intermittent) media errors. In the majority of these cases, at least a portion of the original data can be recovered by repairing the damaged partition table or file system using specialized data recovery software such asTestDisk; software likeddrescuecan image media despite intermittent errors, and image raw data when there is partition table or file system damage. This type of data recovery can be performed by people without expertise in drive hardware as it requires no special physical equipment or access to platters. Sometimes data can be recovered using relatively simple methods and tools;[12]more serious cases can require expert intervention, particularly if parts of files are irrecoverable.Data carvingis the recovery of parts of damaged files using knowledge of their structure. After data has been physically overwritten on a hard disk drive, it is generally assumed that the previous data are no longer possible to recover. In 1996,Peter Gutmann, a computer scientist, presented a paper that suggested overwritten data could be recovered through the use ofmagnetic force microscopy.[13]In 2001, he presented another paper on a similar topic.[14]To guard against this type of data recovery, Gutmann and Colin Plumb designed a method of irreversibly scrubbing data, known as theGutmann methodand used by several disk-scrubbing software packages. Substantial criticism has followed, primarily dealing with the lack of any concrete examples of significant amounts of overwritten data being recovered.[15]Gutmann's article contains a number of errors and inaccuracies, particularly regarding information about how data is encoded and processed on hard drives.[16]Although Gutmann's theory may be correct, there is no practical evidence that overwritten data can be recovered, while research has shown to support that overwritten data cannot be recovered.[specify][17][18][19] Solid-state drives(SSD) overwrite data differently from hard disk drives (HDD) which makes at least some of their data easier to recover. Most SSDs useflash memoryto store data in pages and blocks, referenced bylogical block addresses(LBA) which are managed by theflash translation layer(FTL). When the FTL modifies a sector it writes the new data to another location and updates the map so the new data appear at the target LBA. This leaves the pre-modification data in place, with possibly many generations, and recoverable by data recovery software. Sometimes, data present in the physical drives (Internal/External Hard disk,Pen Drive, etc.) gets lost, deleted and formatted due to circumstances like virus attack, accidental deletion or accidental use of SHIFT+DELETE. In these cases, data recovery software is used to recover/restore the data files. In the list of logical failures of hard disks, a logical bad sector is the most common fault leading data not to be readable. Sometimes it is possible to sidestep error detection even in software, and perhaps with repeated reading and statistical analysis recover at least some of the underlying stored data. Sometimes prior knowledge of the data stored and the error detection and correction codes can be used to recover even erroneous data. However, if the underlying physical drive is degraded badly enough, at least the hardware surrounding the data must be replaced, or it might even be necessary to apply laboratory techniques to the physical recording medium. Each of the approaches is progressively more expensive, and as such progressively more rarely sought. Eventually, if the final, physical storage medium has indeed been disturbed badly enough, recovery will not be possible using any means; the information has irreversibly been lost. Recovery experts do not always need to have physical access to the damaged hardware. When the lost data can be recovered by software techniques, they can often perform the recovery using remote access software over the Internet, LAN or other connection to the physical location of the damaged media. The process is essentially no different from what the end user could perform by themselves.[20] Remote recovery requires a stable connection with an adequate bandwidth. However, it is not applicable where access to the hardware is required, as in cases of physical damage. Usually, there are four phases when it comes to successful data recovery, though that can vary depending on the type of data corruption and recovery required.[21] TheWindowsoperating system can be reinstalled on a computer that is already licensed for it. The reinstallation can be done by downloading the operating system or by using a "restore disk" provided by the computer manufacturer. Eric Lundgren was fined and sentenced to U.S. federal prison in April 2018 for producing 28,000 restore disks and intending to distribute them for about 25 cents each as a convenience to computer repair shops.[22] Data recovery cannot always be done on a running system. As a result, aboot disk,live CD,live USB, or any other type of live distro contains a minimal operating system.	https://en.wikipedia.org/wiki/Data_recovery
Digital inheritanceis the passing down ofdigital assetsto designated (or undesignated)beneficiariesafter a person’s death as part of the estate of the deceased. The process includes understanding what digital assets exist and navigating the rights for heirs to access and use those digital assets after a person has died. Digital media play an increasingly important role in life. The media in which a digitalinheritanceresides can be fully owned by the deceased or under the control of a proprietary service provider. In contrast with physical assets, digital assets are ephemeral and subject to constant change. There are currently many obstacles to successful digital inheritance processes, asestate lawsandprivacy lawsare still catching up with the way modern life is spent in the digital realm. Issues center around user privacy,intellectual propertyrights, and the legal liability of online corporations. With the average person having numerous online accounts, digital inheritance has become a complex issue. Large online service providers are increasingly offering options for users to make decisions on what happens to their data, and who can access it, in the event of their death. Digital inheritance processes can encourage thepreservation of digital contentthat provides both sentimental value to descendants of the deceased and informational value to society's digital heritage. Digital inheritance is the passing down ofdigital assetsto designated (or undesignated)beneficiariesafter a person’s death as part of the estate of the deceased. What was traditionally passed down as physical assets – analog materials such as letters, financial paperwork, photographs, or books – now exist for many people almost entirely in digital form as email, online banking, digital photos, or e-books.[1]In contrast with physical assets, digital assets are ephemeral and subject to constant threats of data corruption, format obsolescence, or licensing restrictions and proprietary control. Digital media can be physically owned, such as those stored on personal computers, hard drives, or optical discs, in which case the digital content exists on a format which can easily be bequeathed and passed down to heirs. A growing majority of digital content and interactions, however, are stored in an online environment and not owned by the individual but by the company providing the online service or product.[2]Examples of this include the online services provided by large corporations such asGoogle,Apple,MicrosoftandFacebook. With the average person having 150 online accounts that require a password,[3]digital inheritance has become a complex legal and ethical issue. Legal conflicts surrounding digital inheritance center around questions ofintellectual propertyrights, user privacy, andestate law. The termdigital estaterefers to the inheritable digital assets included in a person's estate. This includes the digital media itself as well as the rights to have control over that media.[4]A person's digital assets may be digital media that a person owns outright or has the rights to use according to a terms of service agreement. Assets may be stored either online or offline and include online accounts, any form of writing, images and other created static or dynamic content, or any digital content that has economic value. They may include sensitive information, such as banking and medical records, or shared information, such as social media contacts or forums. In contrast with physical assets, digital assets, particularly those stored online, are always vulnerable to change or deletion.[2] Two principal issues arise over a person's digital estate. First, the inheritability of the digital content must be determined. Only digital content for which the deceased holds the copyright may be passed down to an inheritor. There is a distinction in law between full ownership and right-to-use licenses such as in software, digital music, film and books and there is legal precedent for denying resale or bequest of these.[5]Second, the heir or administrator of an estate must be able to access the content. This sometimes means navigating any online contracts or service providers' terms of service agreements regarding their policies on user privacy and user death.[4][6] Estate laws and privacy laws do not yet fully address the challenges presented by the ubiquitous nature of modern life in the digital world. Online service providers, such as those for social media sites, email, andcloud storage services, use terms of service agreements to outline their privacy policies. These are then used in arguments against providing family members access to a deceased user's account.[7]One legal argument makes a distinction between the ownership of an online account and the content that is created on that account: content created by a user, regardless of the platform it is created or displayed on, constitutes that user's intellectual property and should thus be considered inheritable as a digital asset.[8] In the United States, online service providers craft their terms of service agreements to remain compliant with theStored Communications Act(SCA). Originally passed in 1986, the SCA sought to protect communication privacy by prohibiting service providers from disclosing a customer's electronically stored content to a third party. This includes potential beneficiaries of a digital estate.[4] Contracts with service providers may be automatically terminated (by the terms of service) when a customer dies. This may mean that there is no right for heirs to access that data. This is compounded by the fact that many digital assets are only granted with non-transferablerights of use. For example, both Amazon and Apple only offer their digital products with single user rights. This means that digital products bought through such services can only be used by the purchaser, and cannot be passed on.[9] Many solutions to the obstacles faced by digital inheritance have been proposed. One possible solution in the United States calls for a revision to the SCA allowing an exemption for digital estate beneficiaries. This would create less liability for online service providers and allow them to grant a beneficiary access to a deceased user's account as an authorized third party.[7]Another possible solution would be an entirely new federal law regulating the handling of digital assets after death, in which the designated administrator of an estate would receive full access to the deceased user's online accounts.[4]As estate law has traditionally been relegated to states, however, a proposed federal law would be a significant departure from current practice.[7]A third possible solution proposes that online service providers present users with a list of options upon sign-up regarding the disposition of the user's content in case of death. This option would allow users to choose whether or not they desire their content to be preserved and to whom they would grant access, accommodating their right to privacy.[10] As a response to the lack of both federal and state laws concerning digital inheritance, in 2015 theUniform Law Commissionreleased theRevised Uniform Fiduciary Access to Digital Assets Act(RUFADAA).[11]This piece of legislation seeks to balance the interests of digital estate administrators and the privacy concerns of internet account users and service providers. The RUFADAA stipulates that a personal representative (estate administrator,fiduciary, orconservator) of an online account user has the right to access the user's electronic communications if the user had consented to this disclosure either via an online tool (such as Facebook's Legacy Contact feature or Google's Inactive Account Manager) or in a will. If neither of these forms of user consent are on file, an online service provider's terms of service agreement remains in effect and the provider has the right to deny a fiduciary access to electronic communications.[12]As of 2021, 47 states have enacted the RUFADAA.[13] A successfully implemented digital inheritance process has both personal and societal benefits which highlight the concept's importance. In the personal realm, family members' ability to access or receive copies of their deceased loved one's online content, such as social media profiles, blogs, and emails, has real sentimental value and can aid in the grieving process just as much as a person's physical objects. Additionally, any digital content that produced economic value for the original user may continue to do so if passed on to the user's heir(s).[7] Digital inheritance also has beneficial implications for the preservation of society's digital heritage. The passing on of digital estates necessitates that the content of digital assets be preserved either as copies or in its original functioning environment for an heir to receive. Digital inheritance therefore encourages properdigital preservationpractices rather than allowing the content of deceased users to be abandoned and/or eventually deleted. Future generations will then be able to have a better understanding of this society's digital landscape.[7] One method of ensuring that a digital inheritance is handled legally and comprehensively is to create a digital estate plan. This can be an informal plan or legally incorporated into a will in the form ofdigital will. The practical approach to is to keep a regularbackupof digital assets in a secure place and appoint a single person who will postmortem deal with the assets. An up-to-date list of passwords to online accounts would be essential, as well as determining how each online account provider handles data access after a user's death.[3] There are several services that securely store passwords, sending them to designated people after a user's death. Some of these send the customer an email from time to time, prompting to confirm that that person is still alive. Failure to respond to multiple emails makes the service provider assume that the person is deceased and they will disclose the passwords as previously requested. A company may require two verifiers who both must confirm the death, as well as providing a death certificate, before any passwords will be disclosed.[14] There are services that facilitate passing social accounts and digital cryptocurrencies to the beneficiaries after one's passing. They allow users to connect their social accounts, file storage services, and bitcoin wallets to one "vault". The upside of such an approach is that no additional transfer of assets is necessary since transfer is happening on the connected service provider's side, thus keeping risks to the minimum.[15] Social media services have policies and processes to confirm the identity and death of a deceased user. Twitterdoes not allow access to deceased user profiles. They will, however, deactivate an account for someone who is "authorized to act on the behalf of the estate, or with a verified immediate family member of the deceased" provides the user's death certificate and their owngovernment-issued ID.[16] It isFacebook's policy to automatically memorialize a profile if they are made a aware of a user's death. Only verified immediate family members of the deceased may request that the account be fully deleted.[17]If a user would like to decide what happens to their account upon their death, they have two options. In account settings, they can choose to have their account automatically deleted after death, or set up a legacy contact who will have the ability to manage their memorialized page.[18]Facebook will not provide an account's login information to either a legacy contact or a family member.[17] Both Facebook and Twitter have been prey to hoax celebrity death announcements and memorial pages, as well as being entangled in legal battles for the rights to access a departed loved one's social profiles,[19]leading to the need for official action and processes. Google's tool for navigating user death is the Inactive Account Manager. Using this feature, a user can specify a trusted contact that will receive a notification if the user's account has been inactive for a specified amount of time. The user can also decide which data they would like their trusted contact to receive download access to.[20]If a user has not set up a trusted contact in Inactive Account Manager prior to their death, Google will work with families on a case-by-case basis if data from the deceased user's account is requested.[21] Appleusers can add a legacy contact to their Apple ID (this is a new feature included with iOS 15.2, iPadOS 15.2, and macOS 12.1).[22]A legacy contact can access all data stored on a user's iCloud account for up to three years, after which the user's account is deleted. Legacy contacts cannot access any licensed media, in-app purchases, or payment and password information.[23] LinkedInhas a process for removing or memorializing the profiles of deceased members. The request may be initiated by someone with legal authority to act on behalf of the deceased and who has the proper documentation. Non-authorized individuals may simply report a user as deceased.[24] Adigital estate memorialemploys a combination of digital technology,[25]includingonline memorials, for the benefit of the descendant's estate using various forms of communication, data storage and messaging to continue the legacy of the deceased as years progress. This includes services designed to continue contact with descendants with instructions or greetings from the passed. Some organizations ensure that digital inheritance will be protected in the event new technology emerges over periods of generational succession. This would include new applications of the digital inheritance lifestream as technological systems develop.[26]	https://en.wikipedia.org/wiki/Digital_inheritance
IT network assurancequantifiesriskfrom anITnetwork perspective, based on analysis ofnetworkfacts.[1]Examples could be identifying configuration errors innetwork equipment, which may result in loss of connectivity between devices, degradation of performance or network outages. Relevant facts about the network that could be analyzed would include not only networkconfiguration files, but current network state, network traffic analysis, error logs or performance data.[2] Network Assurance is closely related to the topic ofService assurance, which is primarily for service providers and telecommunication networks to ensure the proper level and quality of network service are delivered to customers. IT network assurance is generally for corporate networks and enterprise IT departments.[3]Network assurance involves the engineering process offormal verification, which specifically contrasts with design testing. Verification of network design compares the policy requirements of the network with the actual implementation under all conditions, rather than testing specific test scenarios under a finite number of conditions.[4] A complement to ITapplication-level securityanddata-level securitysolutions, and a sub-set ofNetwork management, Networkassurancemeasures the impact of network change on security, availability, and compliance. Network assurance helps companies keep policies and defences correctly implemented during times of rapid network change. It also helps organizations prioritize remediation efforts and validate network policies and controls.[5][2][6]Colleges are now offering classes specifically for this Network Management sub-domain under IT Forensics.[7]The capabilities of a network assurance solution also overlap closely with the technology category of intent-based networking, which also compares network intent and required policies with actual network designs and configurations to provide assurance of network functions.[2][8][9] IP Fabricintroduced a software platform in 2017 which seeks to improve the reliability and security of enterprise networks through continuous network assurance.Ciscointroduced a network assurance solution in January 2018 called Network Assurance Engine.[9]Veriflow Systems is another vendor that claims to provide network assurance capabilities in its platform through continuous network verification.[10]All three of these systems rely on a mathematical model of the network, rather than monitoring of live network traffic, to analyze and verify behavior in comparison to network intent and desired policies.[11][9] This computing article is astub. You can help Wikipedia byexpanding it.	https://en.wikipedia.org/wiki/IT_network_assurance
Pre-boot authentication(PBA) orpower-on authentication(POA)[1]serves as an extension of theBIOS,UEFIor boot firmware and guarantees a secure, tamper-proof environment external to theoperating systemas a trusted authentication layer. The PBA prevents anything being read from the hard disk such as the operating system until the user has confirmed they have the correct password or other credentials includingmulti-factor authentication.[2] A PBA environment serves as an extension of the BIOS, UEFI or boot firmware and guarantees a secure, tamper-proof environment external to the operating system as a trusted authentication layer.[2]The PBA prevents any operating system from loading until the user has confirmed he/she has the correct password to unlock the computer.[2]That trusted layer eliminates the possibility that one of the millions of lines of OS code can compromise the privacy of personal or company data.[2] in BIOS mode: in UEFI mode: Pre-boot authentication can by performed by an add-on of the operating system like LinuxInitial ramdiskor Microsoft's boot software of the system partition (or boot partition) or by a variety offull disk encryption(FDE) vendors that can be installed separately to the operating system. Legacy FDE systems tended to rely upon PBA as their primary control. These systems have been replaced by systems using hardware-based dual-factor systems likeTPMchips or other proven cryptographic approaches. However, without any form of authentication (e.g. a fully transparent authentication loading hidden keys), encryption provides little protection from advanced attackers as this authentication-less encryption fully rely on the post-boot authentication comes fromActive Directoryauthentication at theGINAstep of Windows. Microsoft released BitLocker Countermeasures[3]defining protection schemes for Windows. For mobile devices that can be stolen and attackers gain permanent physical access (paragraph Attacker with skill and lengthy physical access) Microsoft advise the use of pre-boot authentication and to disable standby power management. Pre-boot authentication can be performed with TPM with PIN protector or any 3rd party FDA vendor. Best security is offered by offloading the cryptographic encryption keys from the protected client and supplying key material externally within the user authentication process. This method eliminates attacks on any built-in authentication method that are weaker than a brute-force attack to the symmetric AES keys used for full disk encryption. Without cryptographic protection of a hardware (TPM) supported secure boot environment, PBA is easily defeated withEvil Maidstyle of attacks. However, with modern hardware (includingTPMor cryptographic multi-factor authentication) most FDE solutions are able to ensure that removal of hardware for brute-force attacks is no longer possible. The standard complement of authentication methods exist for pre-boot authentication including:	https://en.wikipedia.org/wiki/Pre-boot_authentication
Security breach notification lawsordata breach notification lawsarelawsthat require individuals or entities affected by adata breach, unauthorized access to data,[1]to notify their customers and other parties about the breach, as well as take specific steps to remedy the situation based on state legislature. Data breach notification laws have two main goals. The first goal is to allow individuals a chance to mitigate risks against data breaches. The second goal is to promote company incentive to strengthen data security.[2]Together, these goals work to minimize consumer harm from data breaches, including impersonation, fraud, and identity theft.[3] Such laws have been irregularly enacted in all 50U.S. statessince 2002. Currently, all 50 states have enacted forms of data breach notification laws.[4]There is no federal data breach notification law, despite previous legislative attempts.[5]These laws were enacted in response to an escalating number of breaches ofconsumerdatabases containingpersonally identifiable information.[6]Similarly, multiple other countries, like theEuropean UnionGeneral Data Protection Regulation(GDPR) and Australia's Privacy Amendment (Notifiable Data Breaches) Act 2017 (Cth), have added data breach notification laws to combat the increasing occurrences of data breaches.[7] The rise in data breaches conducted by both countries and individuals is evident and alarming, as the number of reported data breaches has increased from 421 in 2011, to 1,091 in 2016, and 1,579 in 2017 according to theIdentity Theft Resource Center(ITRC).[8][9]It has also impacted millions of people and gained increasing public awareness due to large data breaches such as the October 2017 Equifax breach that exposed almost 146 million individual's personal information.[10] On 2018, Australia Privacy Amendment (Notifiable Data Breaches) Act 2017 went into effect.[11]This amended the Privacy Act 1988 (Cth), which had established a notification system for data breaches involving personal information that lead to harm. Now, entities with existing personalinformation securityobligations under the Australian Privacy Act are required to notify the Office of Australian Information Commissioner (OAIC) and affected individuals of all “eligible data breaches.”[12]The amendment is coming off large data breaches experiences in Australia, such as theYahoo hackin 2013 involving thousands of government officials and the data breach of NGOAustralian Red Crossreleasing 550,000 blood donor's personal information. Criticism of the data breach notification include: the unjustified exemption of certain entities such as small businesses and the Privacy Commissioner not required to post data breaches in one permanent place to be used as data for future research. In addition, notification obligations are not consistent at a state level.[13] In mid-2017, China adopted a new Cyber security Law, which included data breach notification requirements.[13] In 1995, theEUpassed theData Protection Directive(DPD), which has recently been replaced with the 2016 General Data Protection Regulation (GDPR), a comprehensive federal data breach notification law. The GDPR offers stronger data protection laws, broader data breach notification laws, and new factors such as the right to data portability. However, certain areas of the data breach notification laws are supplemented by other data security laws.[13] Examples of this include, theEuropean Unionimplemented a breach notification law in theDirective on Privacy and Electronic Communications(E-Privacy Directive) in 2009, specific topersonal dataheld by telecoms and Internet service providers.[14][15]This law contains some of the notification obligations for data breaches.[13] The traffic data of the subscribers, who use voice and data via a network company, is saved from the company only for operational reasons. However, the traffic data must be deleted when they aren’t necessary anymore, in order to avoid the breaches. However, the traffic data is necessary for the creation and treatment of subscriber billing. The use of these data is available only up to the end of the period that the bill can be repaid based on the law of European Union (Article 6 - paragraphs 1-6[16]). Regarding the marketing usage of the traffic data for the sale of additional chargeable services, they can be used from the company only if the subscriber gives his/her consent (but, the consent can be withdrawn at every time). Also, the service provider must inform the subscriber or user of the types of traffic data which are processed and of the duration of that based on the above assumptions. Processing of traffic data, in accordance with the above details, must be restricted to persons acting under the authority of providers of the public communications networks and publicly available electronic communications services handling billing or traffic management, customer enquiries, fraud detection, marketing electronic communications services or providing a value added service, and must be restricted to what is necessary for the purposes of such activities. Data breach notification obligations are included in the new Directive on security of network and information systems (NIS Directive). This creates notification requirements on essential services and digital service providers. Among these include immediately notifying the authorities or computer security incident response teams (CSIRTS) if they experience a significant data breach. Similar to US concerns for a state-by-state approach creating increased costs and difficulty complying with all the state laws, the EU's various breach notification requirements in different laws creates concern.[13] In 2015, Japan amended the Act on the Protection of Personal Information (APPI) to combat massive data leaks. Specifically, the massive Benesse Corporation data leak in 2014 where nearly 29 million pieces of private customer information was leaked and sold. This includes new penal sanctions on illegal transaction, however, there is no specific provision dealing with data breach notification in the APPI. Instead, the Policies Concerning the Protection of Personal Information, in accordance with the APPI, creates a policy that encourages business operators to disclose data breaches voluntarily.[17] Kaori Ishii and Taro Komukai have theorized that the Japanese culture offers a potential explanation for why there is no specific data breach notification law to encourage companies to strengthen data security. The Japanese general public and mass media, in particularly, condemn leaks. Consequently, data leaks quickly result in losing customer trust, brand value, and ultimately profits. An example of this include, after a 2004 data leak, Softbank swiftly lost 107 billion yen and Benesse Corporation lost 940,000 customers after the data leak. This has resulted in compliance with disclosing data leaks in accordance with the policy.[17] While proving the Japanese culture makes specific data breach notification laws necessary is difficult to objectively prove, what has been shown is that companies that experience data breach do experience both financial and reputation harm.[18][19] New Zealand’s Privacy Act 2020 came into force on December 1, 2020, replacing the 1993 act. The act makes notification of privacy breaches mandatory.[20]Organisations receiving and collecting data will now have to report any privacy breach they believe has caused, or is likely to cause, serious harm. Data Breach Notification Laws have been enacted in all 50 states, theDistrict of Columbia,Guam,Puerto Ricoand theVirgin Islands.[6]As of August 2021, attempts to pass a federal data breach notification law have been unsuccessful.[21] The first such law, theCalifornia data security breach notification law,[22]was enacted in 2002 and became effective on July 1, 2003.[23]The bill was enacted in reaction to the fear ofidentity theftand fraud.[8][24]As related in the bill statement, law requires "a state agency, or a person or business that conducts business in California, that owns or licenses computerized data that includes personal information, as defined, to disclose in specified ways, any breach of the security of the data, as defined, to any resident of California whose unencrypted personal information was, or is reasonably believed to have been, acquired by an unauthorized person." In addition, the law permits delayed notification "if a law enforcement agency determines that it would impede a criminal investigation." The law also requires any entity that licenses such information to notify the owner or licensee of the information of any breach of the security of the data. In general, most state laws follow the basic tenets of California's original law: Companies must immediately disclose adata breachto customers, usually in writing.[25]California has since broadened its law to include compromised medical and health insurance information.[26]Where bills differ most is at what level the breach must be reported to the state Attorney General (usually when it affects 500 or 1000 individuals or more). Some states like California publish these data breach notifications on their oag.gov websites. Breaches must be reported if "sensitive personally identifying information has been acquired or is reasonably believed to have been acquired by an unauthorized person, and is reasonably likely to cause substantial harm to the individuals to whom the information relates."[27]This leaves room for some interpretation (will it cause substantial harm?); but breaches of encrypted data need not be reported. Nor must it be reported if data has been obtained or viewed by unauthorized individuals as long as there is no reason to believe they will use the data in harmful ways. TheNational Conference of State Legislaturesmaintains a list of enacted and proposed security breach notification laws.[6]Alabama and South Dakota enacted their data breach notification laws in 2018, making them the final states to do so. Some of the state differences in data breach notification laws include thresholds of harm suffered from data breaches, the need to notify certain law enforcement or consumer credit agencies, broader definitions of personal information, and differences in penalties for non-compliance.[13] As of August 2021, there is no federal data breach notification law. The first proposed federal data breach notification law was introduced to Congress in 2003, but it never exited the Judiciary Committee.[5]Similarly, a number of bills that would establish a national standard for data security breach notification have been introduced in theU.S. Congress, but none passed in the109th Congress.[28]In fact, in 2007, three federal data breach notification laws were proposed, but none passed Congress.[5]In his 2015 State of the Union speech,President Obamaproposed new legislation to create a national data breach standard that would establish a 30-day notification requirement from the discovery of a breach.[29]This led to President Obama's 2015 Personal Data Notification & Protection Act (PDNPA) proposal. This would have created federal notification guidelines and standards, but it never came out of committee.[5] Chlotia Garrison and Clovia Hamilton theorized that a potential reason for the inability to pass a federal law on data breach notifications is states' rights. As of now, all 50 states have varying data breach notification laws. Some are restrictive, while others are broad.[5]While there is not a comprehensive federal law on data breach notifications, some federal laws require notifications of data breaches in certain circumstances. Some notable examples include: theFederal Trade Commission Act(FTC Act), theFinancial Services Modernization Act (Gramm-Leach-Bliley Act), and theHealth Insurance Portability and Accountability Act(HIPAA).[13] Most scholars, like Angela Daly, advocate for federal data breach notification laws emphasize the problem with having varying forms of data breach notification laws. That is, companies are forced to comply with multiple state data breach notification laws. This creates increased difficulty to comply with the laws and the costs. In addition, scholars have argued that a state-by-state approach has created the problem of uncompensated victims and inadequate incentives to persuade companies and governments to invest in data security.[13] Advocates of a state-by-state approach to data breach notification laws emphasize increased efficiency, increased incentives to have the local governments increase data security, limited federal funding available due to multiple projects, and lastly states are able to quickly adapt and pass laws to constantly evolving data breach technologies.[10]In 2018, a majority ofstate attorneys generalopposed a proposed federal data breach notification law that wouldpreemptstate laws.[30] Data breaches occur because of technical issues like bad code to economic issues causing competing firm to not cooperate with each other to tackle data security.[31]In response, data breach notification laws attempt to prevent harm to companies and the public. A serious harm of data breaches isidentity theft. Identity theft can harm individuals when their personal data is stolen and is used by another party to create financial harm such as withdrawing their money, non financially such as fraudulently claiming their health benefits, and pretending to be them and committing crimes.[32]Based on data collected from 2002 to 2009 from theU.S. Federal Trade Commission, the use of data breach notification has helped to decrease identity theft by 6.1 percent.[33] Overall, data breach notifications leads to decreasing market value, evident in publicly traded companies experiencing a decrease in market valuation.[34][35]Other costs include loss of consumer confidence and trust in the company, loss of business, decreased productivity, and exposure to third-party liability.[35]Notably, the type of data that is leaked from the breach has varying economic impact. A data breach that leaks sensitive data experiences harsher economic repercussions.[36] Most federal data breach lawsuits share certain characteristics. These include a plaintiff seeking relief from the loss of an identity theft, emotional distress, future losses, and increased risk of future harm; the majority of litigation are private class actions; the defendants are usually large firms or businesses; a mix of common law and statutory causes of action; and lastly most cases settle or are dismissed.[37]	https://en.wikipedia.org/wiki/Security_breach_notification_laws
Transparent data encryption(often abbreviated toTDE) is a technology employed byMicrosoft,IBMandOracletoencryptdatabasefiles. TDE offers encryption at file level. TDE enables the encryption ofdata at rest, encrypting databases both on the hard drive and consequently onbackupmedia. It does not protectdata in transitnordata in use. Enterprises typically employ TDE to solve compliance issues such asPCI DSSwhich require the protection of data at rest. Microsoft offers TDE as part of itsMicrosoft SQL Server2008, 2008 R2, 2012, 2014, 2016, 2017 and 2019.[1]TDE was only supported on the Evaluation, Developer, Enterprise and Datacenter editions of Microsoft SQL Server, until it was also made available in the Standard edition for 2019.[2]SQL TDE is supported byhardware security modulesfrom Thales e-Security, Townsend Security and SafeNet, Inc. IBM offers TDE as part ofDb2as of version 10.5 fixpack 5.[3]It is also supported in cloud versions of the product by default, Db2 on Cloud and Db2 Warehouse on Cloud. Oracle requires theOracle Advanced Securityoption for Oracle 10g and 11g to enable TDE.[citation needed]Oracle TDE addresses encryption requirements associated with public and private privacy and security mandates such as PCI andCaliforniaSB 1386. Oracle Advanced Security TDE column encryption was introduced in Oracle Database 10g Release 2. Oracle Advanced Security TDE tablespace encryption and support forhardware security modules(HSMs) were introduced with Oracle Database 11gR1. Keys for TDE can be stored in an HSM to manage keys across servers, protect keys with hardware, and introduce a separation of duties. The same key is used to encrypt columns in a table, regardless of the number of columns to be encrypted. These encryption keys are encrypted using the database server master key and are stored in a dictionary table in the database. SQL Server utilizes an encryption hierarchy that enables databases to be shared within a cluster or migrated to other instances without re-encrypting them. The hierarchy consists of a combination of symmetric and asymmetric ciphers:[4] During database backups,compressionoccurs after encryption. Due to the fact that strongly encrypted data cannot be significantly compressed, backups of TDE encrypted databases require additional resources. To enable automatic booting, SQL Server stores the lowest level encryption keys in persistent storage (using theDPAPIstore). This presents a potential security issue because the stored keys can be directly recovered from a live system or from backups and used to decrypt the databases.[5]	https://en.wikipedia.org/wiki/Transparent_data_encryption
Secure USB flash drivesprotect the data stored on them from access by unauthorized users.USB flash driveproducts have been on the market since 2000, and their use is increasing exponentially.[1][2]As businesses have increased demand for these drives, manufacturers are producing faster devices with greaterdata storagecapacities. An increasing number of portable devices are used in business and decreased numbers for consumers, such aslaptops,notebooks,personal digital assistants(PDA),smartphones,USB flash drivesand other mobile devices. Companies in particular are at risk when sensitive data are stored on unsecuredUSB flash drivesby employees who use the devices to transport data outside the office. The consequences of losing drives loaded with such information can be significant, including the loss of customer data, financial information, business plans and other confidential information, with the associated risk of reputation damage. USB flash drives pose two major challenges to information system security: data leakage owing to their small size and ubiquity and system compromise through infections fromcomputer viruses,malwareandspyware. The large storage capacity of USB flash drives relative to their small size and low cost means that using them for data storage without adequate operational and logical controls may pose a serious threat to information availability, confidentiality and integrity. The following factors should be taken into consideration for securing important assets: The average cost of a data breach from any source (not necessarily a flash drive) ranges from less than $100,000 to about $2.5 million.[1] ASanDisksurvey[3]characterized the data corporate end users most frequently copy: Examples of security breaches resulting from USB drives include: In the early days of computer viruses, malware, and spyware, the primary means of transmission and infection was thefloppy disk. Today, USB flash drives perform the same data and software storage and transfer role as the floppy disk, often used to transfer files between computers which may be on different networks, in different offices, or owned by different people. This has made USB flash drives a leading form of information system infection. When a piece of malware gets onto a USB flash drive, it may infect the devices into which that drive is subsequently plugged. The prevalence of malware infection by means of USB flash drive was documented in a 2011 Microsoft study[6]analyzing data from more than 600 million systems worldwide in the first half of 2011. The study found that 26 percent of all malware infections of Windows system were due to USB flash drives exploiting theAutoRunfeature inMicrosoft Windows. That finding was in line with other statistics, such as the monthly reporting of most commonly detected malware by antivirus company ESET, which lists abuse of autorun.inf as first among the top ten threats in 2011.[7] The Windows autorun.inf file contains information on programs meant to run automatically when removable media (often USB flash drives and similar devices) are accessed by a Windows PC user. The default Autorun setting in Windows versions prior to Windows 7 will automatically run a program listed in the autorun.inf file when you access many kinds of removable media. Many types of malware copy themselves to removable storage devices: while this is not always the program's primary distribution mechanism, malware authors often build in additional infection techniques. Examples of malware spread by USB flash drives include: Since the security of the physical drive cannot be guaranteed without compromising the benefits of portability, security measures are primarily devoted to making the data on a compromised drive inaccessible to unauthorized users and unauthorized processes, such as may be executed by malware. One common approach is to encrypt the data for storage and routinely scan USB flash drives for computer viruses, malware and spyware with anantivirusprogram, although other methods are possible. Software solutions such asBitLocker,DiskCryptorand the popularVeraCryptallow the contents of a USB drive to be encrypted automatically and transparently. Also,Windows 7Enterprise, Windows 7 Ultimate andWindows Server 2008 R2provide USB drive encryption usingBitLockerto Go. TheApple ComputerMac OS Xoperating system has provided software for disc data encryption sinceMac OS X Pantherwas issued in 2003 (see also:Disk Utility).[citation needed] Additional software can be installed on an external USB drive to prevent access to files in case the drive becomes lost or stolen. Installing software on company computers may help track and minimize risk by recording the interactions between any USB drive and the computer and storing them in a centralized database.[citation needed] Some USB drives utilize hardware encryption in which microchips within the USB drive provide automatic and transparent encryption.[8]Some manufacturers offer drives that require a pin code to be entered into a physical keypad on the device before allowing access to the drive. The cost of these USB drives can be significant but is starting to fall due to this type of USB drive gaining popularity. Hardware systems may offer additional features, such as the ability to automatically overwrite the contents of the drive if the wrong password is entered more than a certain number of times. This type of functionality cannot be provided by a software system since the encrypted data can simply be copied from the drive. However, this form of hardware security can result in data loss if activated accidentally by legitimate users and strong encryption algorithms essentially make such functionality redundant. As the encryption keys used in hardware encryption are typically never stored in the computer's memory,technicallyhardware solutions are less subject to "cold boot" attacks than software-based systems.[9]In reality however, "cold boot" attacks pose little (if any) threat, assuming basic, rudimentary, security precautions are taken with software-based systems. The security of encrypted flash drives is constantly tested by individual hackers as well as professional security firms. At times (as in January 2010) flash drives that have been positioned as secure were found to have been poorly designed such that they provide little or no actual security, giving access to data without knowledge of the correct password.[10] Flash drives that have been compromised (and claimed to now be fixed) include: All of the above companies reacted immediately. Kingston offered replacement drives with a different security architecture. SanDisk, Verbatim, and Trek released patches. In commercial environments, where most secure USB drives are used,[1]a central/remote management system may provide organizations with an additional level of IT asset control, significantly reducing the risks of a harmful data breach. This can include initial user deployment and ongoing management, password recovery, data backup, remote tracking of sensitive data and termination of any issued secure USB drives. Such management systems are available assoftware as a service(SaaS), where Internet connectivity is allowed, or as behind-the-firewall solutions. SecureData, Inc offers a software free Remote Management Console that runs from a browser. By using an app on a smartphone, Admins can manage who, when and where USB devices were last accessed with a complete audit trail. Used by Hospitals, large enterprises, Universities and the federal government to track access and protect data in transit and at rest.	https://en.wikipedia.org/wiki/USB_flash_drive_security
AByzantine faultis a condition of a system, particularly adistributed computingsystem, where a fault occurs such that different symptoms are presented to different observers, including imperfect information on whether a system component has failed. The term takes its name from anallegory, the "Byzantine generals problem",[1]developed to describe a situation in which, to avoid catastrophic failure of a system, the system's actors must agree on a strategy, but some of these actors are unreliable in such a way as to cause other (good) actors to disagree on the strategy and they may be unaware of the disagreement. A Byzantine fault is also known as aByzantine generals problem, aByzantine agreement problem, or aByzantine failure. Byzantine fault tolerance(BFT) is the resilience of afault-tolerant computer systemor similar system to such conditions. A Byzantine fault is any fault presenting different symptoms to different observers.[2]A Byzantine failure is the loss of a system service due to a Byzantine fault in systems that requireconsensusamong multiple components.[3] The Byzantine allegory considers a number of generals who are attacking a fortress. The generals must decide as a group whether to attack or retreat; some may prefer to attack, while others prefer to retreat. The important thing is that all generals agree on a common decision, for a halfhearted attack by a few generals would become arout, and would be worse than either a coordinated attack or a coordinated retreat. The problem is complicated by the presence of treacherous generals who may not only cast a vote for a suboptimal strategy; they may do so selectively. For instance, if nine generals are voting, four of whom support attacking while four others are in favor of retreat, the ninth general may send a vote of retreat to those generals in favor of retreat, and a vote of attack to the rest. Those who received a retreat vote from the ninth general will retreat, while the rest will attack (which may not go well for the attackers). The problem is complicated further by the generals being physically separated and having to send their votes via messengers who may fail to deliver votes or may forge false votes. Without message signing, Byzantine fault tolerance can only be achieved if the total number of generals is greater than three times the number of disloyal (faulty) generals. There can be a default vote value given to missing messages. For example, missing messages can be given a"null" value. Further, if the agreement is that the null votes are in the majority, a pre-assigned default strategy can be used (e.g., retreat).[4] The typical mapping of this allegory onto computer systems is that the computers are the generals and their digital communication system links are the messengers. Although the problem is formulated in the allegory as a decision-making and security problem, in electronics, it cannot be solved bycryptographicdigital signaturesalone, because failures such as incorrect voltages can propagate through the encryption process. Thus, a faulty message could be sent such that some recipients detect the message as faulty (bad signature), others see it is having a good signature, and a third group also sees a good signature but with different message contents than the second group.[2] The problem of obtaining Byzantine consensus was conceived and formalized byRobert Shostak, who dubbed it theinteractive consistencyproblem. This work was done in 1978 in the context of the NASA-sponsored SIFT[5]project in the Computer Science Lab atSRI International. SIFT (for Software Implemented Fault Tolerance) was the brainchild of John Wensley, and was based on the idea of using multiple general-purpose computers that would communicate through pairwise messaging in order to reach a consensus, even if some of the computers were faulty. At the beginning of the project, it was not clear how many computers in total were needed to guarantee that a conspiracy ofnfaulty computers could not "thwart" the efforts of the correctly-operating ones to reach consensus. Shostak showed that a minimum of 3n+1 are needed, and devised a two-round 3n+1messaging protocol that would work forn=1. His colleague Marshall Pease generalized the algorithm for any n > 0, proving that 3n+1 is both necessary and sufficient. These results, together with a later proof byLeslie Lamportof the sufficiency of 3nusing digital signatures, were published in the seminal paper,Reaching Agreement in the Presence of Faults.[6]The authors were awarded the 2005Edsger W. Dijkstra Prizefor this paper. To make the interactive consistency problem easier to understand, Lamport devised a colorful allegory in which a group of army generals formulate a plan for attacking a city. In its original version, the story cast the generals as commanders of theAlbanianarmy. The name was changed, eventually settling on "Byzantine", at the suggestion of Jack Goldberg to future-proof any potential offense-giving.[7]This formulation of the problem, together with some additional results, were presented by the same authors in their 1982 paper, "The Byzantine Generals Problem".[4] The objective of Byzantine fault tolerance is to be able to defend against failures of system components with or without symptoms that prevent other components of the system from reaching an agreement among themselves, where such an agreement is needed for the correct operation of the system. The remaining operationally correct components of a Byzantine fault tolerant system will be able to continue providing the system's service as originally intended, assuming there are a sufficient number of accurately-operating components to maintain the service. When considering failure propagation only via errors, Byzantine failures are considered the most general and most difficult class of failures among thefailure modes. The so-called fail-stop failure mode occupies the simplest end of the spectrum. Whereas the fail-stop failure mode simply means that the only way to fail is anodecrash, detected by other nodes, Byzantine failures imply no restrictions on what errors can be created, which means that a failed node can generate arbitrary data, including data that makes it appear like a functioning node to a subset of other nodes. Thus, Byzantine failures can confuse failure detection systems, which makes fault tolerance difficult. Despite the allegory, a Byzantine failure is not necessarily asecurityproblem involving hostile human interference: it can arise purely from physical or software faults. The terms fault and failure are used here according to the standard definitions[8]originally created by a joint committee on "Fundamental Concepts and Terminology" formed by theIEEEComputer Society's Technical Committee on Dependable Computing and Fault-Tolerance andIFIPWorking Group 10.4 on Dependable Computing and Fault Tolerance.[9]See alsodependability. Byzantine fault tolerance is only concerned with broadcast consistency, that is, the property that when a component broadcasts a value to all the other components, they all receive exactly this same value, or in the case that the broadcaster is not consistent, the other components agree on a common value themselves. This kind of fault tolerance does not encompass the correctness of the value itself; for example, an adversarial component that deliberately sends an incorrect value, but sends that same value consistently to all components, will not be caught in the Byzantine fault tolerance scheme. Several early solutions were described by Lamport, Shostak, and Pease in 1982.[4]They began by noting that the Generals' Problem can be reduced to solving a "Commander and Lieutenants" problem where loyal Lieutenants must all act in unison and that their action must correspond to what the Commander ordered in the case that the Commander is loyal: There are many systems that claim BFT without meeting the above minimum requirements (e.g., blockchain). Given that there is mathematical proof that this is impossible, these claims need to include a caveat that their definition of BFT strays from the original. That is, systems such as blockchain don't guarantee agreement. They use resource-intensive mechanisms that make disagreements impractical to maintain. Several system architectures were designed c. 1980 that implemented Byzantine fault tolerance. These include: Draper's FTMP,[13]Honeywell's MMFCS,[14]and SRI's SIFT.[5] In 1999, Miguel Castro andBarbara Liskovintroduced the "Practical Byzantine Fault Tolerance" (PBFT) algorithm,[15]which provides high-performance Byzantine state machine replication, processing thousands of requests per second with sub-millisecond increases in latency. After PBFT, several BFT protocols were introduced to improve its robustness and performance. For instance, Q/U,[16]HQ,[17]Zyzzyva,[18]and ABsTRACTs,[19]addressed the performance and cost issues; whereas other protocols, like Aardvark[20]and RBFT,[21]addressed its robustness issues. Furthermore, Adapt[22]tried to make use of existing BFT protocols, through switching between them in an adaptive way, to improve system robustness and performance as the underlying conditions change. Furthermore, BFT protocols were introduced that leverage trusted components to reduce the number of replicas, e.g., A2M-PBFT-EA[23]and MinBFT.[24] Several examples of Byzantine failures that have occurred are given in two equivalent journal papers.[2][3]These and other examples are described on theNASADASHlink web pages.[25] Byzantine fault tolerance mechanisms use components that repeat an incoming message (or just its signature, which can be reduced to just a single bit of information if self-checking pairs are used for nodes) to other recipients of that incoming message. All these mechanisms make the assumption that the act of repeating a message blocks the propagation of Byzantine symptoms. For systems that have a high degree of safety or security criticality, these assumptions must be proven to be true to an acceptable level offault coverage. When providing proof through testing, one difficulty is creating a sufficiently wide range of signals with Byzantine symptoms.[26]Such testing will likely require specializedfault injectors.[27][28] Byzantine errors were observed infrequently and at irregular points during endurance testing for the newly constructedVirginiaclass submarines, at least through 2005 (when the issues were publicly reported).[29] TheBitcoin networkworks in parallel to generate ablockchainwithproof-of-workallowing the system to overcome Byzantine failures and reach a coherent global view of the system's state.[30][31]Someproof of stakeblockchains also use BFT algorithms.[32] Byzantine Fault Tolerance (BFT) is a crucial concept inblockchain technology, ensuring that a network can continue to function even when some nodes[33](participants) fail or act maliciously. This tolerance is necessary because blockchains are decentralized systems with no central authority, making it essential to achieve consensus among nodes, even if some try to disrupt the process. Safety Mechanisms:Different blockchains use various BFT-based consensus mechanisms like Practical Byzantine Fault Tolerance (PBFT), Tendermint, andDelegated Proof of Stake (DPoS)to handle Byzantine faults. These protocols ensure that the majority of honest nodes can agree on the next block in the chain, securing the network against attacks and preventingdouble-spendingand other types of fraud. Practical examples of networks includeHyperledger Fabric,CosmosandKleverin this sequence. 51% Attack Mitigation:While traditional blockchains like Bitcoin use Proof of Work (PoW), which is susceptible to a51% attack, BFT-based systems are designed to tolerate up to one-third of faulty or malicious nodes without compromising the network's integrity. Decentralized Trust:Byzantine Fault Tolerance underpins the trust model indecentralizednetworks. Instead of relying on a central authority, the network's security depends on the ability of honest nodes to outnumber and outmaneuver malicious ones. Private and Permissioned Blockchains:BFT is especially important in private or permissioned blockchains, where a limited number of known participants need to reach a consensus quickly and securely. These networks often use BFT protocols to enhance performance and security. Some aircraft systems, such as the Boeing 777Aircraft Information Management System(via itsARINC659 SAFEbus network), the Boeing 777 flight control system, and the Boeing 787 flight control systems, use Byzantine fault tolerance; because these are real-time systems, their Byzantine fault tolerance solutions must have very low latency. For example, SAFEbus can achieve Byzantine fault tolerance within the order of a microsecond of added latency.[34][35][36]TheSpaceX Dragonconsiders Byzantine fault tolerance in its design.[37]	https://en.wikipedia.org/wiki/Byzantine_fault_tolerance
Control reconfigurationis an active approach incontrol theoryto achievefault-tolerant controlfordynamic systems.[1]It is used when severefaults, such as actuator or sensor outages, cause a break-up of thecontrol loop, which must be restructured to preventfailureat the system level. In addition to loop restructuring, thecontrollerparameters must be adjusted to accommodate changed plant dynamics. Control reconfiguration is a building block toward increasing thedependabilityof systems underfeedbackcontrol.[2] The figure to the right shows a plant controlled by a controller in a standard control loop. The nominal linear model of the plant is {x˙=Ax+Buy=Cx{\displaystyle {\begin{cases}{\dot {\mathbf {x} }}&=\mathbf {A} \mathbf {x} +\mathbf {B} \mathbf {u} \\\mathbf {y} &=\mathbf {C} \mathbf {x} \end{cases}}} The plant subject to a fault (indicated by a red arrow in the figure) is modelled in general by {x˙f=Afxf+Bfuyf=Cfxf{\displaystyle {\begin{cases}{\dot {\mathbf {x} }}_{f}&=\mathbf {A} _{f}\mathbf {x} _{f}+\mathbf {B} _{f}\mathbf {u} \\\mathbf {y} _{f}&=\mathbf {C} _{f}\mathbf {x} _{f}\end{cases}}} where the subscriptf{\displaystyle f}indicates that the system is faulty. This approach models multiplicative faults by modified system matrices. Specifically, actuator faults are represented by the new input matrixBf{\displaystyle \mathbf {B} _{f}}, sensor faults are represented by the output mapCf{\displaystyle \mathbf {C} _{f}}, and internal plant faults are represented by the system matrixAf{\displaystyle \mathbf {A} _{f}}. The upper part of the figure shows a supervisory loop consisting offault detection and isolation(FDI) andreconfigurationwhich changes the loop by To this end, the vectors of inputs and outputs containall available signals, not just those used by the controller in fault-free operation. Alternative scenarios can model faults as an additive external signalf{\displaystyle \mathbf {f} }influencing the state derivatives and outputs as follows: {x˙f=Axf+Bu+Efyf=Cfxf+Ff{\displaystyle {\begin{cases}{\dot {\mathbf {x} }}_{f}&=\mathbf {A} \mathbf {x} _{f}+\mathbf {B} \mathbf {u} +\mathbf {E} \mathbf {f} \\\mathbf {y} _{f}&=\mathbf {C} _{f}\mathbf {x} _{f}+\mathbf {F} \mathbf {f} \end{cases}}} The goal of reconfiguration is to keep the reconfigured control-loop performance sufficient for preventing plant shutdown. The following goals are distinguished: Internal stability of the reconfigured closed loop is usually the minimum requirement. The equilibrium recovery goal (also referred to as weak goal) refers to the steady-state output equilibrium which the reconfigured loop reaches after a given constant input. This equilibrium must equal the nominal equilibrium under the same input (as time tends to infinity). This goal ensures steady-state reference tracking after reconfiguration. The output trajectory recovery goal (also referred to as strong goal) is even stricter. It requires that the dynamic response to an input must equal the nominal response at all times. Further restrictions are imposed by the state trajectory recovery goal, which requires that the state trajectory be restored to the nominal case by the reconfiguration under any input. Usually a combination of goals is pursued in practice, such as the equilibrium-recovery goal with stability. The question whether or not these or similar goals can be reached for specific faults is addressed byreconfigurabilityanalysis. This paradigm aims at keeping the nominal controller in the loop. To this end, a reconfiguration block can be placed between the faulty plant and the nominal controller. Together with the faulty plant, it forms the reconfigured plant. The reconfiguration block has to fulfill the requirement that the behaviour of the reconfigured plant matches the behaviour of the nominal, that is fault-free plant.[3] In linear model following, a formal feature of the nominal closed loop is attempted to be recovered. In the classical pseudo-inverse method, the closed loop system matrixA¯=A−BK{\displaystyle {\bar {\mathbf {A} }}=\mathbf {A} -\mathbf {B} \mathbf {K} }of a state-feedback control structure is used. The new controllerKf{\displaystyle \mathbf {K} _{f}}is found to approximateA¯{\displaystyle {\bar {\mathbf {A} }}}in the sense of an induced matrix norm.[4] In perfect model following, a dynamic compensator is introduced to allow for the exact recovery of the complete loop behaviour under certain conditions. In eigenstructure assignment, the nominal closed loop eigenvalues and eigenvectors (the eigenstructure) is recovered to the nominal case after a fault. Optimisation control schemes include: linear-quadratic regulator design (LQR), model predictive control (MPC) and eigenstructure assignment methods.[5] Some probabilistic approaches have been developed.[6] There are learning automata, neural networks, etc.[7] The methods by which reconfiguration is achieved differ considerably. The following list gives an overview of mathematical approaches that are commonly used.[8] Prior to control reconfiguration, it must be at least determined whether a fault has occurred (fault detection) and if so, which components are affected (fault isolation). Preferably, a model of the faulty plant should be provided (fault identification). These questions are addressed byfault diagnosismethods. Fault accommodationis another common approach to achievefault tolerance. In contrast to control reconfiguration, accommodation is limited to internal controller changes. The sets of signals manipulated and measured by the controller are fixed, which means that the loop cannot be restructured.[9]	https://en.wikipedia.org/wiki/Control_reconfiguration
Inengineering,damage toleranceis a property of a structure relating to its ability to sustain defects safely until repair can be effected. The approach toengineering designto account for damage tolerance is based on the assumption that flaws can exist in any structure and such flaws propagate with usage. This approach is commonly used inaerospace engineering,mechanical engineering, andcivil engineeringto manage the extension ofcracksin structure through the application of the principles offracture mechanics. A structure is considered to be damage tolerant if a maintenance program has been implemented that will result in the detection and repair of accidental damage,corrosionandfatiguecracking before such damage reduces theresidual strengthof the structure below an acceptable limit. Structures upon which human life depends have long been recognized as needing an element offail-safety. When describing his flying machine,Leonardo da Vincinoted that "In constructing wings one should make one chord to bear the strain and a looser one in the same position so that if one breaks under the strain, the other is in the position to serve the same function."[1] Prior to the 1970s, the prevailing engineering philosophy of aircraft structures was to ensure that airworthiness was maintained with a single part broken, a redundancy requirement known asfail-safety. However, advances infracture mechanics, along with infamous catastrophic fatigue failures such as those in thede Havilland Cometprompted a change in requirements for aircraft. It was discovered that a phenomenon known asmultiple-site damagecould cause many small cracks in the structure, which grow slowly by themselves, to join one another over time, creating a much larger crack, and significantly reducing the expected time until failure[2] Not all structure must demonstrate detectable crack propagation to ensure safety of operation. Some structures operate under thesafe-life designprinciple, where an extremely low level ofriskis accepted through a combination of testing and analysis that the part will never form a detectable crack due to fatigue during the service life of the part. This is achieved through a significant reduction of stresses below the typical fatigue capability of the part. Safe-life structures are employed when the cost or infeasibility of inspections outweighs the weight penalty and development costs associated with safe-life structures.[1]An example of a safe-life component is thehelicopter rotorblade. Due to the extremely large numbers of cycles endured by the rotating component, an undetectable crack may grow to a critical length in a single flight and before the aircraft lands, result in a catastrophic failure that regular maintenance could not have prevented. In ensuring the continued safe operation of the damage tolerant structure, inspection schedules are devised. This schedule is based on many criteria, including: These factors affect how long the structure may operate normally in the damaged condition before one or more inspection intervals has the opportunity to discover the damaged state and effect a repair. The interval between inspections must be selected with a certain minimum safety, and also must balance the expense of the inspections, the weight penalty of lowering fatigue stresses, and the opportunity costs associated with a structure being out of service for maintenance. Manufacturers and operators of aircraft, trains, and civil engineering structures like bridges have a financial interest in ensuring that the inspection schedule is as cost-efficient as possible. In the example of aircraft, because these structures are often revenue producing, there is anopportunity costassociated with the maintenance of the aircraft (lost ticket revenue), in addition to the cost of maintenance itself. Thus, this maintenance is desired to be performed infrequently, even when such increased intervals cause increased complexity and cost to the overhaul. Crack growth, as shown byfracture mechanics, is exponential in nature; meaning that the crack growth rate is a function of an exponent of the current crack size (seeParis' law). This means that only the largest cracks influence the overall strength of a structure; small internal damages do not necessarily decrease the strength. A desire for infrequent inspection intervals, combined with the exponential growth of cracks in structure has led to the development ofnon-destructive testingmethods which allow inspectors to look for very tiny cracks which are often invisible to the naked eye. Examples of this technology includeeddy current,ultrasonic,dye penetrant, andX-rayinspections. By catching structural cracks when they are very small, and growing slowly, these non-destructive inspections can reduce the amount of maintenance checks, and allow damage to be caught when it is small, and still inexpensive to repair. As an example, such repair can be achieved by drilling a small hole at the crack tip, thus effectively turning the crack into akeyhole-notch.[3]	https://en.wikipedia.org/wiki/Damage_tolerance
In computermain memory,auxiliary storageandcomputer buses,data redundancyis the existence of data that is additional to the actual data and permits correction of errors in stored or transmitted data. The additional data can simply be a complete copy of the actual data (a type ofrepetition code), or only select pieces of data that allowdetection of errors and reconstructionof lost or damaged data up to a certain level. For example, by including computed check bits,ECC memoryis capable of detecting and correcting single-bit errors within eachmemory word, whileRAID 1combines twohard disk drives(HDDs) into a logical storage unit that allows stored data to survive a complete failure of one drive.[1][2]Data redundancy can also be used as a measure againstsilent data corruption; for example,file systemssuch asBtrfsandZFSuse data andmetadatachecksumming in combination with copies of stored data to detect silent data corruption and repair its effects.[3] While different in nature,data redundancyalso occurs indatabase systemsthat have values repeated unnecessarily in one or more records orfields, within atable, or where the field is replicated/repeated in two or more tables. Often this is found inunnormalizeddatabase designs and results in the complication of database management, introducing the risk of corrupting the data, and increasing the required amount ofstorage. When done on purpose from a previously normalized database schema, itmaybe considered a form ofdatabase denormalization; used to improve performance of database queries (shorten the database response time). For instance, when customer data are duplicated and attached with each product bought, then redundancy of data is a known source ofinconsistencysince a given customer might appear with different values for one or more of their attributes.[4]Data redundancy leads todata anomalies and corruptionand generally should be avoided by design;[5]applyingdatabase normalizationprevents redundancy and makes the best possible usage of storage.[6]	https://en.wikipedia.org/wiki/Data_redundancy
Defence in depth(also known asdeep defenceorelastic defence) is amilitary strategythat seeks to delay rather than prevent the advance of an attacker, buying time and causing additionalcasualtiesby yielding space. Rather than defeating an attacker with a single, strong defensive line, defence in depth relies on the tendency of an attack to lose momentum over time or as it covers a larger area. A defender can thus yield lightly defended territory in an effort to stress an attacker'slogisticsor spread out a numerically superior attacking force. Once an attacker has lost momentum or is forced to spread out topacifya large area, defensivecounter-attackscan be mounted on the attacker's weak points, with the goal being to causeattritionor drive the attacker back to its original starting position. A conventional defence strategy would concentrate all military resources at afront line, which, if breached by an attacker, would leave the remaining defenders in danger of beingoutflankedand surrounded and would leavesupply lines,communications, and command vulnerable. Defence in depth requires that a defender deploy their resources, such asfortifications, field works andmilitary unitsat and well behind the front line. Although attackers may find it easier to breach the more weakly defended front line, as they advance, they continue to meet resistance. As they penetrate deeper, their flanks become vulnerable, and, should the advance stall, they risk beingenveloped. The defence in depth strategy is particularly effective against attackers who are able toconcentrate their forcesand attack a small number of places on an extended defensive line. Defenders that can fall back to a succession of preparedpositionscan extract a high price from the advancing enemy while themselves avoiding the danger of being overrun or outflanked. Delaying the enemy advance mitigates the attacker's advantage of surprise and allows time to move defending units to make a defence and to prepare a counter-attack. A well-planned defence in depth strategy will deploy forces in mutually supportive positions and in appropriate roles. For example,poorly trained troopsmay be deployed instatic defencesat the front line, whereas better trained and equipped troops form a mobile reserve. Successive layers of defence may use different technologies against various targets; for example,dragon's teethmight present a challenge fortanksbut is easily circumvented byinfantry, while another barrier ofwire entanglementshas the opposite effects on the respective forces. Defence in depth may allow a defender to maximise the defensive possibilities of naturalterrainand other advantages. The disadvantages of defence in depth are that it may be unacceptable for a defender to plan to give ground to an attacker. This may be because vital military or economic resources are close to the front line or because yielding to an enemy is unacceptable for political or cultural reasons. In addition, the continuousretreatsthat are required by defence in depth require the defender to have a high degree ofmobilityin order to retreat successfully, and they assume that the defender'smoralewill recover from the retreat. A possible early example of this came at theBattle of Cannaein 216 BC, whenHannibalemployed this manoeuvre in order toencircleand destroy eightRoman legions, but that is disputed by some historians.[1] Edward Luttwakused the term to describe his theory ofthe defensive strategyemployed by theLate Roman armyin the 3rd and 4th centuries AD. Later examples of defence in depth might be Europeanhill fortsand the development ofconcentric castles. These castles utilized many layers, including ditches, outer walls, towers, inner walls, and a keep, with some layers like the outer ditch intended to only slow attackers and reduce their coordination. In those examples, the inner layers of defence can support the outer layers withprojectilefire and an attacker must breach each line of defence in turn with the prospect of significant losses, and the defenders have the option of falling back to fight again. On a strategic level, defence in depth was employed by theByzantine military. In theAmerican Revolutionary War'sBattle of Cowpens, the American forces were positioned in three lines which soaked up theshockof the Britishchargeand inflicted heavy casualties before the Americans were able to overrun the British who, at this point, had lost theircohesion. More recent examples of defence in depth include the multiple lines of trenches of theFirst World Warand the followingTurkish War of Independencewhere the Turks stopped the advance of the Greeks towards Ankara. Also plans for thedefence of Britain against a potential German invasionin theSecond World War. During theBattle of Normandy,Wehrmachtforces utilized thebocageof the area, flooding of fields, and strategic placement of defences to create successive lines of defences to slow the attackingAlliesin hopes that reinforcements would arrive. ThePacific Theatrealso had many examples of defence in depth, with the Japanese inflicting heavy casualties on the Americans in the Battles ofTarawa,Saipan,Peleliu,Iwo Jima, andOkinawa. The best modern example of a successful defence in depth is that of theBattle of Kurskin the Second World War. During the battle, theRed Armydeliberately drew the Germans into an attritional battle in multiple, well-prepared defensive lines, before launching massive counter-attacks on either side of the 9th Army in the north and the 4th Panzer Army in the south. The initial Germanoffensivenever fully penetrated the Red Army lines. By contrast, the subsequent Red Armycounter-offensivepushed the front line hundreds of miles westwards. Colonel Francis J. Kelly discussed the employment of defence in depth inArmy Special Forcescamps during theVietnam War. Kelly, a former U.S. Army Special Forces commander and author ofVietnam Studies U.S. Army Special Forces 1961–1971, stated in his work that the austere Special Forces fighting camps were highly functional and easily defended.[citation needed] While untested, this was also the plannedNATOstrategy in Europe during theCold Warat theFulda Gap.[citation needed]However, by the 1980s,West Germanyeventually pressured NATO to abandon this orthodox doctrine as it would have entailed allowing the country to be overrun byWarsaw Pactforces before they would be finally stopped. Instead, NATO agreed to an alternative doctrine of "Forward defence," which was criticized for not only being militarily senseless that would have quickly forced NATO to resort totactical nuclear weaponswhen Warsaw Pact broke through with their considerable conventional forces, but also that enemy considered the doctrine so provocative and potentially aggressive that striking first seemed a viable option as a result.[2] The concept of defence-in-depth (DiD) is also applied in the fields of life-threatening technologies where it is critical to avoid a disaster, or to save lives. The safety ofnuclear reactorsandradioactive waste repositoriesalso fundamentally relies on multiple systems and redundant barriers. The principle of redundancy is essential to prevent the occurrence of dramatic failures and in case where a failure would develop to retard the progression of a potentially disastrous event and to give extra time for taking again the control of the failed system. Ultimately, if a failure cannot be avoided, DiD also contributes to mitigate the consequences and to attenuate the negative impacts of the failure. Defence-in-depth is required to guarantee the robustness of vital systems, e.g. innuclear technologiesand foraerospacesystems where safety is critical. The DiD approach can be applied to any sensitive technology: submarines and naval systems, biotechnology, pharmaceutic industry, informatics, bank and financial systems, etc. In nature, theimmune systemof most evolved organisms also appeals to multiple lines of defence in case apathogenwould defeat the first line of defence ofcells,tissuesandorgans. The robustness of thescientific methodalso relies on multiple lines of evidence and multiple lines of reasoning: strong claims require strong and multiple evidence. Therepeatabilityand thereproducibilityof experimental and calculations results are essential to guarantee their robustness and correctness. This associated with the scientific questioning and a constant interrogative attitude is at the core of the self-correcting process guiding the science. Academics from Oxford University'sFuture of Humanity Institutealso applied the concept of defence in depth to designing strategies for the prevention ofexistential catastrophes, especially those involvinghuman extinction.[3]	https://en.wikipedia.org/wiki/Defence_in_depth
Inecology,resilienceis the capacity of anecosystemto respond to a perturbation ordisturbanceby resisting damage and subsequently recovering. Such perturbations and disturbances can includestochasticevents such asfires,flooding,windstorms, insect population explosions, and human activities such asdeforestation, fracking of the ground for oil extraction, pesticide sprayed in soil, and theintroduction of exoticplant or animal species. Disturbances of sufficientmagnitudeor duration can profoundly affect an ecosystem and may force an ecosystem to reach athresholdbeyond which a differentregimeof processes and structures predominates.[2]When such thresholds are associated with a critical orbifurcation point, theseregime shiftsmay also be referred to ascritical transitions.[3] Human activities that adversely affect ecological resilience such asreduction of biodiversity,exploitation of natural resources,pollution,land use, andanthropogenic climate changeare increasingly causing regime shifts in ecosystems, often to less desirable and degraded conditions.[2][4]Interdisciplinary discourse on resilience now includes consideration of the interactions of humans and ecosystems via socio-ecological systems, and the need for shift from themaximum sustainable yieldparadigm toenvironmental resource managementandecosystem management, which aim to build ecological resilience through "resilience analysis, adaptive resource management, and adaptive governance".[2][5]Ecological resilience has inspired other fields and continues to challenge the way they interpret resilience, e.g.supply chain resilience. According to French engineerJean-François Jaudon, a lowecological footprinton an environment, thereby respecting itsbiocapacity, allows for greater resilience of thebiotope. This opinion is shared by researcherNiko Paech. Furthermore, naturalrewildingis not sufficient in cases where the biotope has suffered a large ecological footprint; it must be organized and resembleecological restorationorrenaturationto preservebiodiversity. TheIPCC Sixth Assessment Reportdefines resilience as, “not just the ability to maintain essential function, identity and structure, but also the capacity for transformation.” The IPCC considers resilience both in terms of ecosystem recovery as well as the recovery and adaptation of human societies to natural disasters.[6] The concept of resilience in ecological systems was first introduced by the Canadian ecologistC.S. Holling[7]in order to describe the persistence of natural systems in the face of changes in ecosystem variables due to natural or anthropogenic causes. Resilience has been defined in two ways in ecological literature: The second definition has been termed ‘ecological resilience’, and it presumes the existence of multiple stable states or regimes.[8] For example, some shallow temperate lakes can exist within either clear water regime, which provides manyecosystem services, or a turbid water regime, which provides reduced ecosystem services and can producetoxic algae blooms. The regime or state is dependent upon lakephosphorus cycles, and either regime can be resilient dependent upon the lake's ecology and management.[1][2] Likewise,Mulgawoodlands of Australia can exist in a grass-rich regime that supports sheep herding, or a shrub-dominated regime of no value for sheep grazing.Regime shiftsare driven by the interaction offire,herbivory, and variable rainfall. Either state can be resilient dependent upon management.[1][2] EcologistsBrian Walker,C S Hollingand others describe four critical aspects of resilience:latitude,resistance,precariousness, andpanarchy. The first three can apply both to a whole system or the sub-systems that make it up. Closely linked to resilience isadaptive capacity, which is the property of an ecosystem that describes change in stability landscapes and resilience.[8]Adaptive capacity in socio-ecological systems refers to the ability of humans to deal with change in their environment by observation, learning and altering their interactions.[2] Resilience refers to ecosystem's stability and capability of tolerating disturbance and restoring itself. If the disturbance is of sufficient magnitude or duration, a threshold may be reached where the ecosystem undergoes aregime shift, possibly permanently. Sustainable use of environmental goods and services requires understanding and consideration of the resilience of the ecosystem and its limits. However, the elements which influence ecosystem resilience are complicated. For example, various elements such as thewater cycle, fertility,biodiversity, plant diversity and climate, interact fiercely and affect different systems. There are many areas where human activity impacts upon and is also dependent upon the resilience of terrestrial, aquatic and marine ecosystems. These include agriculture, deforestation, pollution, mining, recreation, overfishing, dumping of waste into the sea and climate change. Agriculturecan be used as a significant case study in which the resilience of terrestrial ecosystems should be considered. Theorganic matter(elements carbon and nitrogen) in soil, which is supposed to be recharged by multiple plants, is the main source of nutrients forcropgrowth.[9]In response to global food demand andshortages, however,intensive agriculturepractices including the application ofherbicidesto control weeds, fertilisers to accelerate and increase crop growth andpesticidesto control insects, reduceplant biodiversitywhile the supply of organic matter to replenishsoil nutrientsand preventsurface runoffis diminished. This leads to a reduction insoil fertilityand productivity.[9]More sustainable agricultural practices would take into account and estimate the resilience of the land and monitor and balance the input and output of organic matter. The termdeforestationhas a meaning that covers crossing the threshold of forest's resilience and losing its ability to return to its originally stable state. To recover itself, a forest ecosystem needs suitable interactions among climate conditions and bio-actions, and enough area. In addition, generally, the resilience of a forest system allows recovery from a relatively small scale of damage (such as lightning or landslide) of up to 10 percent of its area.[10]The larger the scale of damage, the more difficult it is for the forest ecosystem to restore and maintain its balance. Deforestation also decreases biodiversity of both plant and animal life and can lead to an alteration of the climatic conditions of an entire area. According to theIPCC Sixth Assessment Report, carbon emissions due to land use andland use changespredominantly come from deforestation, thereby increasing the long-term exposure of forest ecosystems to drought and other climate change-induced damages.[11]Deforestation can also lead to species extinction, which can have a domino effect particularly when keystone species are removed or when a significant number of species is removed and their ecological function is lost.[4][12] It has been estimated by the United Nations Food and Agriculture Organisation that over 70% of the world'sfish stocksare either fully exploited or depleted which meansoverfishingthreatensmarine ecosystemresilience and this is mostly by rapid growth of fishing technology.[15]One of the negative effects on marine ecosystems is that over the last half-century the stocks ofcoastal fishhave had a huge reduction as a result of overfishing for its economic benefits.[16]Blue fin tunais at particular risk of extinction. Depletion of fish stocks results in lowered biodiversity and consequently imbalance in the food chain, and increased vulnerability to disease. In addition to overfishing, coastal communities are suffering the impacts of growing numbers of large commercialfishing vesselsin causing reductions of small local fishing fleets. Many local lowland rivers which are sources of fresh water have become degraded because of the inflows of pollutants and sediments.[17] Dumping both depends upon ecosystem resilience whilst threatening it. Dumping of sewage and other contaminants into the ocean is often undertaken for the dispersive nature of the oceans and adaptive nature and ability formarine lifeto process themarine debrisand contaminants. However, waste dumping threatens marine ecosystems by poisoning marine life andeutrophication. According to the International Maritime Organisationoil spillscan have serious effects on marine life. The OILPOL Convention recognized that most oil pollution resulted from routine shipboard operations such as the cleaning of cargo tanks. In the 1950s, the normal practice was simply to wash the tanks out with water and then pump the resulting mixture of oil and water into the sea. OILPOL 54 prohibited the dumping of oily wastes within a certain distance from land and in 'special areas' where the danger to the environment was especially acute. In 1962 the limits were extended by means of an amendment adopted at a conference organized by IMO. Meanwhile, IMO in 1965 set up a Subcommittee on Oil Pollution, under the auspices of its Maritime Safety committee, to address oil pollution issues.[18] The threat of oil spills to marine life is recognised by those likely to be responsible for the pollution, such as the International Tanker Owners Pollution Federation: The marine ecosystem is highly complex and natural fluctuations inspecies composition, abundance and distribution are a basic feature of its normal function. The extent of damage can therefore be difficult to detect against this background variability. Nevertheless, the key to understanding damage and its importance is whether spill effects result in a downturn in breeding success, productivity, diversity and the overall functioning of the system. Spills are not the only pressure on marine habitats; chronic urban and industrial contamination or the exploitation of the resources they provide are also serious threats.[19] The Woods Hole Oceanographic Institution calls nutrient pollution the most widespread, chronic environmental problem in the coastal ocean. The discharges of nitrogen, phosphorus, and other nutrients come from agriculture, waste disposal, coastal development, and fossil fuel use. Once nutrient pollution reaches the coastal zone, it stimulates harmful overgrowths of algae, which can have direct toxic effects and ultimately result in low-oxygen conditions. Certain types of algae are toxic. Overgrowths of these algae result in harmfulalgal blooms, which are more colloquially referred to as "red tides" or "brown tides". Zooplankton eat the toxic algae and begin passing the toxins up the food chain, affecting edibles like clams, and ultimately working their way up to seabirds, marine mammals, and humans. The result can be illness and sometimes death.[20] There is increasing awareness that a greater understanding and emphasis of ecosystem resilience is required to reach the goal ofsustainable development.[17][21][22]A similar conclusion is drawn by Perman et al. who use resilience to describe one of 6 concepts ofsustainability; "A sustainable state is one which satisfies minimum conditions for ecosystem resilience through time".[23]Resilience science has been evolving over the past decade, expanding beyond ecology to reflect systems of thinking in fields such aseconomicsandpolitical science.[24]And, as more and more people move into densely populated cities, using massive amounts of water, energy, and other resources, the need to combine these disciplines to consider the resilience ofurban ecosystemsand cities is of paramount importance.[25] The interdependence of ecological and social systems has gained renewed recognition since the late 1990s by academics including Berkes and Folke[26]and developed further in 2002 by Folke et al.[1]As the concept of sustainable development has evolvedbeyond the 3 pillarsof sustainable development to place greater political emphasis on economic development. This is a movement which causes wide concern in environmental and social forums and which Clive Hamilton describes as "the growth fetish".[27] The purpose of ecological resilience that is proposed is ultimately about averting our extinction as Walker cites Holling in his paper: "[..] "resilience is concerned with [measuring] the probabilities of extinction” (1973, p. 20)".[28]Becoming more apparent in academic writing is the significance of the environment and resilience in sustainable development.[24]Folke et al state that the likelihood of sustaining development is raised by "Managing for resilience"[1]whilst Perman et al. propose that safeguarding the environment to "deliver a set of services" should be a "necessary condition for an economy to be sustainable".[23]The growing application of resilience to sustainable development has produced a diversity of approaches and scholarly debates.[24] The challenge of applying the concept of ecological resilience to the context of sustainable development is that it sits at odds with conventional economic ideology and policy making. Resilience questions thefree marketmodel within which global markets operate. Inherent to the successful operation of a free market is specialisation which is required to achieve efficiency and increase productivity. This very act of specialisation weakens resilience by permitting systems to become accustomed to and dependent upon their prevailing conditions. In the event of unanticipated shocks; this dependency reduces the ability of the system to adapt to these changes.[1]Correspondingly; Perman et al. note that; "Some economic activities appear to reduce resilience, so that the level of disturbance to which the ecosystem can be subjected to without parametric change taking place is reduced".[23] Berkes and Folke table a set of principles to assist with "building resilience and sustainability" which consolidate approaches ofadaptive management, local knowledge-based management practices and conditions for institutional learning and self-organisation.[26] More recently, it has been suggested by Andrea Ross that the concept of sustainable development is no longer adequate in assisting policy development fit for today's global challenges and objectives. This is because the concept of sustainable development is "based onweak sustainability" which doesn't take account of the reality of "limits to earth's resilience".[29]Ross draws on theimpact of climate changeon the global agenda as a fundamental factor in the "shift towards ecological sustainability" as an alternative approach to that of sustainable development.[29] Because climate change is a major and growing driver ofbiodiversity loss, and that biodiversity and ecosystem functions and services, significantly contribute toclimate change adaptation, mitigation and disaster risk reduction, proponents ofecosystem-based adaptationsuggest that the resilience of vulnerable human populations and the ecosystem services upon which they depend are critical factors for sustainable development in a changing climate. Scientific research associated with resilience is beginning to play a role in influencing policy-making and subsequent environmental decision making. This occurs in a number of ways: Ecological resilience and the thresholds by which resilience is defined are closely interrelated in the way that they influence environmental policy-making, legislation and subsequently environmental management. The ability of ecosystems to recover from certain levels of environmental impact is not explicitly noted in legislation, however, because of ecosystem resilience, some levels of environmental impact associated with development are made permissible by environmental policy-making and ensuing legislation. Some examples of the consideration of ecosystem resilience within legislation include: The theoretical basis for many of the ideas central to climate resilience have actually existed since the 1960s. Originally an idea defined for strictly ecological systems,resiliencein ecology was initially outlined byC.S. Hollingas the capacity for ecological systems and relationships within those systems to persist and absorb changes to "state variables, driving variables, and parameters."[38]This definition helped form the foundation for the notion of ecologicalequilibrium: the idea that the behavior of natural ecosystems is dictated by a homeostatic drive towards some stable set point. Under this school of thought (which maintained quite a dominant status during this time period), ecosystems were perceived to respond to disturbances largely throughnegative feedbacksystems – if there is a change, the ecosystem would act to mitigate that change as much as possible and attempt to return to its prior state. As greater amounts of scientific research in ecological adaptation and natural resource management was conducted, it became clear that oftentimes, natural systems were subjected to dynamic, transient behaviors that changed how they reacted to significant changes in state variables: rather than work back towards a predetermined equilibrium, the absorbed change was harnessed to establish a new baseline to operate under. Rather than minimize imposed changes, ecosystems could integrate and manage those changes, and use them to fuel the evolution of novel characteristics. This new perspective of resilience as a concept that inherently works synergistically with elements ofuncertaintyandentropyfirst began to facilitate changes in the field ofadaptive managementand environmental resources, through work whose basis was built by Holling and colleagues yet again.[39][40] By the mid 1970s, resilience began gaining momentum as an idea inanthropology,culture theory, and othersocial sciences. There was significant work in these relatively non-traditional fields that helped facilitate the evolution of the resilience perspective as a whole. Part of the reason resilience began moving away from an equilibrium-centric view and towards a more flexible, malleable description of social-ecological systems was due to work such as that of Andrew Vayda andBonnie McCayin the field of social anthropology, where more modern versions of resilience were deployed to challenge traditional ideals of cultural dynamics.[41]	https://en.wikipedia.org/wiki/Ecological_resilience
Elegant degradationis a term used in engineering to describe what occurs to machines which are subject to constant, repetitive stress. Externally, such a machine maintains the same appearance to the user, appearing to function properly. Internally, the machine slowly weakens over time. Unable to withstand the stress, it eventually breaks down.[1]Compared tograceful degradation, the operational quality does not decrease at all, but the breakdown may be just as sudden.[2] This term's meaning varies depending on context and field, and may not be strictly considered exclusive to engineering. For instance, this is used as a mechanism in the food industry as applied in the degradation oflignin,cellulose,pentosan, andpolymers, among others.[3]The concept is also used to extract chemicals such as the elegant degradation ofPaederusfuscipesto obtainpederinand hemiacetal pseuodopederin.[4]In this process degradation is induced by heat. A play with the same name also used it as a metaphor for the current state of the world.[1][5] This engineering-related article is astub. You can help Wikipedia byexpanding it.	https://en.wikipedia.org/wiki/Elegant_degradation
Anerror-tolerant design(orhuman-error-tolerant design[1]) is one that does not unduly penalize user orhuman errors. It is the human equivalent offault tolerant designthat allows equipment to continue functioning in the presence of hardware faults, such as a "limp-in" mode for anautomobileelectronics unit that would be employed if something like theoxygen sensorfailed. Use of forcing functions orbehavior-shaping constraintsis one technique in error-tolerant design. An example is theinterlockor lockout of reverse in thetransmissionof a moving car. This prevents errors, and prevention of errors is the most effective technique in error-tolerant design. The practice is known aspoka-yokein Japan where it was introduced byShigeo Shingoas part of theToyota Production System. The next most effective technique in error-tolerant design is the mitigation or limitation of the effects of errors after they have been made. An example is a checking or confirmation function such as an"Are you sure" dialog boxwith the harmless option preselected in computersoftwarefor an action that could have severe consequences if made in error, such as deleting or overwriting files (although the consequence of inadvertent file deletion has been reduced from theDOSdays by a concept like thetrash caninMac OS, which has been introduced in mostGUIinterfaces). Adding too great a mitigating factor in some circumstances can become a hindrance, where the confirmation becomes mechanical this may become detrimental - for example, if a prompt is asked for every file in a batch delete, one may be tempted to simply agree to each prompt, even if a file is deleted accidentally. Another example isGoogle's use ofspell checkingon searches performed through their search engine. The spell checking minimises the problems caused by incorrect spelling by not only highlighting the error to the user, but by also providing a link to search using the correct spelling instead. Searches like this are commonly performed using a combination ofedit distance,soundex, andmetaphonecalculations.	https://en.wikipedia.org/wiki/Error-tolerant_design
Human erroris an action that has been done but that was "not intended by the actor; not desired by a set of rules or an external observer; or that led the task or system outside its acceptable limits".[1]Human error has been cited as a primary cause and contributing factor in disasters and accidents in industries as diverse asnuclear power(e.g., theThree Mile Island accident),aviation,space exploration(e.g., theSpace Shuttle Challenger disasterandSpace Shuttle Columbia disaster), andmedicine. Prevention of human error is generally seen as a major contributor toreliabilityandsafetyof (complex) systems. Human error is one of the many contributing causes ofriskevents. Human error refers to something having been done that was "not intended by the actor; not desired by a set of rules or an external observer; or that led the task or system outside its acceptable limits".[1]In short, it is a deviation from intention, expectation or desirability.[1]Logically, human actions can fail to achieve their goal in two different ways: the actions can go as planned, but the plan can be inadequate (leading to mistakes); or, the plan can be satisfactory, but the performance can be deficient (leading toslipsandlapses).[2][3]However, a mere failure is not an error if there had been no plan to accomplish something in particular.[1] Human error and performance are two sides of the same coin: "human error" mechanisms are the same as "human performance" mechanisms; performance later categorized as 'error' is done so in hindsight:[3][4]therefore actions later termed "human error" are actually part of the ordinary spectrum of human behaviour. The study ofabsent-mindednessin everyday life provides ample documentation and categorization of such aspects of behavior. While human error is firmly entrenched in the classical approaches to accident investigation and risk assessment, it has no role in newer approaches such asresilience engineering.[5] There are many ways to categorize human error:[6][7] Thecognitivestudy of human error is a very active research field, including work related to limits ofmemoryandattentionand also todecision makingstrategies such as theavailability heuristicand othercognitive biases. Such heuristics and biases are strategies that are useful and often correct, but can lead to systematic patterns of error. Misunderstandings as a topic in human communication have been studied inconversation analysis, such as the examination of violations of thecooperative principleand Gricean maxims. Organizational studies of error or dysfunction have included studies ofsafety culture. One technique for analyzing complex systems failure that incorporates organizational analysis ismanagement oversight risk tree analysis.[13][14][15] Some researchers have argued that the dichotomy of human actions as "correct" or "incorrect" is a harmfuloversimplificationof acomplex phenomenon.[16][17]A focus on the variability of human performance and how human operators (and organizations) can manage that variability, may be a more fruitful approach. Newer approaches, such as resilience engineering mentioned above, highlight the positive roles that humans can play in complex systems. In resilience engineering, successes (things that go right) and failures (things that go wrong) are seen as having the same basis, namely human performance variability. A specific account of that is theefficiency–thoroughness trade-off principle,[18]which can be found on all levels of human activity, in individuals as well as in groups.	https://en.wikipedia.org/wiki/Human_error
Inengineering, afail-safeis a design feature or practice that, in the event of afailureof the design feature, inherently responds in a way that will cause minimal or no harm to other equipment, to the environment or to people. Unlikeinherent safetyto a particular hazard, a system being "fail-safe" does not mean that failure is naturally inconsequential, but rather that the system's design prevents or mitigates unsafe consequences of the system's failure. If and when a "fail-safe" system fails, it remains at least as safe as it was before the failure.[1][2]Since many types of failure are possible,failure mode and effects analysisis used to examine failure situations and recommend safety design and procedures.[3] Some systems can never be made fail-safe, as continuous availability is needed.Redundancy,fault tolerance, orcontingency plansare used for these situations (e.g. multiple independently controlled and fuel-fed engines).[4] Examples include: Examples include: As well as physical devices and systems fail-safe procedures can be created so that if a procedure is not carried out or carried out incorrectly no dangerous action results. For example: Fail-safe (foolproof) devices are also known aspoka-yokedevices.Poka-yoke, aJapaneseterm, was coined byShigeo Shingo, a quality expert.[11][12]"Safe to fail" refers to civil engineering designs such as theRoom for the River project in Netherlandsand the Thames Estuary 2100 Plan[13][14]which incorporate flexible adaptation strategies orclimate change adaptationwhich provide for, and limit, damage, should severe events such as 500-year floods occur.[15] Fail-safeandfail-secureare distinct concepts.Fail-safemeans that a device will not endanger lives or property when it fails.Fail-secure,also calledfail-closed,means that access or data will not fall into the wrong hands in a security failure. Sometimes the approaches suggest opposite solutions. For example, if a building catches fire, fail-safe systems would unlock doors to ensure quick escape and allow firefighters inside, while fail-secure would lock doors to prevent unauthorized access to the building. The opposite offail-closedis calledfail-open. Fail active operational can be installed on systems that have a high degree of redundancy so that a single failure of any part of the system can be tolerated (fail active operational) and a second failure can be detected – at which point the system will turn itself off (uncouple, fail passive). One way of accomplishing this is to have three identical systems installed, and a control logic which detects discrepancies. An example for this are many aircraft systems, among theminertial navigation systemsandpitot tubes. During theCold War, "failsafe point" was the term used for the point of no return for AmericanStrategic Air Commandnuclear bombers, just outside Soviet airspace. In the event of receiving an attack order, the bombers were required to linger at the failsafe point and wait for a second confirming order; until one was received, they would not arm their bombs or proceed further.[16]The design was to prevent any single failure of the American command system causing nuclear war. This sense of the term entered the American popular lexicon with the publishing of the 1962 novelFail-Safe. (Other nuclear war command control systems have used the opposite scheme,fail-deadly, which requires continuous or regular proof that an enemy first-strike attack hasnotoccurred topreventthe launching of a nuclear strike.)	https://en.wikipedia.org/wiki/Fail-safe
Indistributed computing,failure semanticsis used to describe and classifyerrorsthat distributed systems can experience.[1][2] A list of types of errors that can occur:	https://en.wikipedia.org/wiki/Failure_semantics
Fall backis a feature of amodem protocolindata communicationwhereby two communicatingmodemswhich experiencedata corruption(due to linenoise, for example) can renegotiate with each other to use a lower-speed connection.Fall forwardis a corresponding feature whereby two modems which have "fallen back" to a lower speed can later return to the higher speed if the connection improves. A common featuremodemswas the concept of fallback, allowing them to talk to less-capable modems. During the call initiation the modem would play a series of signals into the line and wait for the remote modem to "answer" them. They would start at high speeds and progressively get slower and slower until they heard an answer. Thus, two USR modems would be able to connect at 9600 bit/s, but, when a user with a 2400-bit/s modem called in, the USR would "fall back" to the common 2400-bit/s speed. This would also happen if a V.32 modem and a HST modem were connected. Because they used a different standard at 9600 bit/s, they would fall back to their highest commonly supported standard at 2400 bit/s. The same applies to V.32bis and 14400 bit/s HST modem, which would still be able to communicate with each other at only 2400 bit/s. A modem that can fall back in order to communicate with an older, slower modem is an example ofbackward compatibility, while one that can fall back in order to communicate when the line becomes noisy is an example ofgraceful degradation. Article based onfall backandfall forwardatFOLDOC, used withpermission.	https://en.wikipedia.org/wiki/Fall_back_and_forward
Agraceful exit[1](orgraceful handling) is a simpleprogramming idiom[citation needed]wherein aprogramdetects a seriouserrorcondition and "exits gracefully" in a controlled manner as a result. Often the program prints a descriptiveerror messageto aterminalorlogas part of the graceful exit. Usually, code for a graceful exit exists when the alternative — allowing the error to go undetected andunhandled— would produce spurious errors or later anomalous behavior that would be more difficult for theprogrammertodebug. The code associated with a graceful exit may also take additional steps, such as closingfiles, to ensure that the program leaves data in a consistent, recoverable state. Graceful exits are not always desired. In many cases, an outrightcrashcan give the software developer the opportunity to attach a debugger or collect important information, such as acore dumporstack trace, to diagnose the root cause of the error. In a language that supports formalexception handling, a graceful exit may be the final step in the handling of an exception. In other languages graceful exits can be implemented with additional statements at the locations of possible errors. The phrase "graceful exit" has also been generalized to refer to letting go from a job or relationship in life that has ended.[2][3] In thePerlprogramming language, graceful exits are generally implemented via thedieoperator. For example, the code for opening a file often reads like the following: If the attempt to open the filemyresultsfails, the containing program will terminate with an error message and anexit statusindicating abnormal termination. In theJavaprogramming language, thetry...catchblock is used often to catchexceptions. All potentially dangerous code is placed inside the block and, if an exception occurred, is stopped, or caught. InCone can use theerror(3)function, provided inGNUby theGNU C Library. If the first parameter is non-zero this function will exit from the parent process and return that parameter. Thiscomputer-programming-related article is astub. You can help Wikipedia byexpanding it.	https://en.wikipedia.org/wiki/Graceful_exit
Intrusion toleranceis afault-tolerant designapproach to defending information systems against malicious attacks. In that sense, it is also acomputer securityapproach. Abandoning the conventional aim of preventing all intrusions, intrusion tolerance instead calls for triggering mechanisms that prevent intrusions from leading to a system security failure. Indistributed computingthere are two major variants of intrusion tolerance mechanisms: mechanisms based on redundancy, such as theByzantine fault tolerance, as well as mechanisms based on intrusion detection as implemented inintrusion detection system) and intrusion reaction. Intrusion-tolerance has started to influence the design of server architectures in academic institutions, andindustry. Examples of such server architectures include KARMA,SplunkIT Service Intelligence (ITSI), project ITUA, and thepractical Byzantine Fault Tolerance(pBFT) model.[1]	https://en.wikipedia.org/wiki/Intrusion_tolerance
Progressive enhancementis a strategy inweb designthat puts emphasis onweb contentfirst, allowingeveryone to accessthe basic content and functionality of a web page, whileuserswith additional browser features or faster Internet access receive the enhanced version instead. This strategy speeds up loading and facilitates crawling byweb search engines, as text on a page is loaded immediately through theHTMLsource code rather than having to wait for JavaScript to initiate and load the content subsequently, meaning content ready for consumption "out of the box" is served immediately, and not behind additional layers.[1][2][3] This strategy involvesseparatingthepresentation semanticsfrom the content, with presentation being implemented in one or more optional layers, activated based on aspects of thebrowserorInternetconnection of theclient. In practice, this means serving content throughHTML, the "lowest common denominator" of web standards, and applying styling and animation throughCSSto the technically possible extent, then applying further enhancements throughJavaScript.[4]DeprecatedAdobe Flashcould be thought of as having shared the final spot with JavaScript while it was widely in use.[5][6]Since web browsers can load site features to the extent supported rather than failing to load the entire site due to one error or missing feature in JavaScript, a progressively enhancing site is more stable andbackwards compatible.[4] "Progressive enhancement" was coined by Steven Champeon andNick Finckat theSXSWInteractive conference on March 11, 2003, in Austin,[7]and through a series of articles forWebmonkeywhich were published between March and June 2003.[8] SpecificCascading Style Sheets(CSS) techniques pertaining to flexibility of the page layout accommodating different screen resolutions is the concept associated with theresponsive web designapproach. In 2012,net Magazinechose Progressive Enhancement as #1 on its list of Top Web Design Trends for 2012 (responsive designwas #2).[9]Googlehas encouraged the adoption of progressive enhancement to help "our systems (and a wider range of browsers) see usable content and basic functionality when certain web design features are not yet supported".[10] The strategy is an evolution of a previous web design strategy known asgraceful degradation, wherein Web pages were designed for the latest browsers first, but then made to work well in older versions of browser software.[11][3]Graceful degradation aims to allow a page to "degrade" – to remain presentable andaccessibleeven if certain technologies expected by the design are absent.[12] In progressive enhancement the strategy is deliberately reversed: Theweb contentis created with amarkup document, geared towards the lowest common denominator of browser software functionality.[13][8][12]If content is to be revealed interactively through JavaScript, such as a collapsible navigation menu, the HTML markup reveals all the content by default and JavaScript itself hides some of the content.[3]Thedeveloperadds all desired functionality to the presentation and behavior of the page, using modern CSS,Scalable Vector Graphics(SVG), orJavaScript.[14] The progressive enhancement approach is derived from Champeon's early experience (c.1993–1994) withStandard Generalized Markup Language(SGML), predating HTML and other Web presentation languages.[8] Writing content with semantic markup and considering the presentation of the content separately, rather than being embedded in the markup itself, is a concept referred to as the rule ofseparation of presentation and content[15][16]. Champeon expressed a hope that, sinceweb browsersprovide a default presentation style for HTML content, this would result in websites with their content written as semantic HTML, leaving presentation choice to the web browser.[8]However, the needs of web designers led to theHTMLstandard being extended withhardcodedfeatures that allowed HTML content to prescribe specific styles, and taking away this option from consumers and their web browsers.[8]These features forced publishers to choose between adopting a newdisruptive technologiesor allowing content to remain accessible to audiences that used other browsers, a dilemma between design and compatibility.[8][17][12]During the 1990s, an increasing number of websites would not work in anything but the latest versions of popular browsers.[12] This trend reversed after the 1990s, once CSS was widely supported,[8][17]through grassroots educational efforts (from Eric Costello, Owen Briggs, Dave Shea, and others) showing Web designers how to use CSS for layout purposes.[citation needed] The progressive enhancement strategy consists of the following core principles:[8] Web pages created according to the principles of progressive enhancement are by their nature more accessible,[27]backwards compatible,[6]andoutreaching, because the strategy demands that basic content always be available, not obstructed by features or scripting that may be easily disabled, unsupported (e.g. bytext-based web browsers), or blocked on computers in sensitive environments.[14]Additionally, the sparse markup principle makes it easier for tools that read content aloud to find that content. It is unclear as to how well progressive enhancement sites work with older tools designed to deal with table layouts, "tag soup", and the like.[citation needed] Theclient (computing)is able to select which parts of a page to download beyond basic HTML (e.g. styling, images, etc.), and can optonly to download parts necessaryfor desired usage to speed up loading and reduce bandwidth and power consumption. For example, a client may choose to only download basic HTML, without loading style sheets, scripts, and media (e.g. images), due to low internet speeds caused by geographical location, poor cellular signal, or throttled speed due to exhausted high-speed data plan. This also reduces bandwidth consumption on the server side. In comparison, pages whose initial content is loaded through AJAX require the client to inefficiently run JavaScript to download and view page content, rather than downloading the content immediately. Improved results with respect tosearch engine optimization(SEO) is another side effect of a progressive enhancement-based Web design strategy. Because the basic content is always accessible to search engine spiders, pages built with progressive enhancement methods avoid problems that may hinder search engine indexing, whereas having to render the basic page content through JavaScript execution would make crawling slow and inefficient.[27][30] Some skeptics, such as Garret Dimon, have expressed their concern that progressive enhancement is not workable in situations that rely heavily on JavaScript to achieve certain user interface presentations or behaviors.[31]Laurie Gray (Information Architect atKnowledgeStorm[32]) countered with the point that informational pages should be coded using progressive enhancement in order to be indexed by search engine spiders.[33]Geoff Stearns (author ofSWFObject, a popular Flash application) argued that Flash-heavy pages should be coded using progressive enhancement.[34] Designers Douglas Bowman and Bob Stein expressed doubts concerning the principle of the separation of content and presentation in absolute terms, pushing instead for a realistic recognition that the two are inextricably linked.[35][15]	https://en.wikipedia.org/wiki/Progressive_enhancement
High availability(HA) is a characteristic of a system that aims to ensure an agreed level of operational performance, usuallyuptime, for a higher than normal period.[1] There is now more dependence on these systems as a result of modernization. For instance, in order to carry out their regular daily tasks, hospitals and data centers need their systems to be highly available.Availabilityrefers to the ability of the user community to obtain a service or good, access the system, whether to submit new work, update or alter existing work, or collect the results of previous work. If a user cannot access the system, it is – from the user's point of view –unavailable.[2]Generally, the termdowntimeis used to refer to periods when a system is unavailable. High availability is a property ofnetworkresilience, the ability to "provide and maintain an acceptable level of service in the face offaultsand challenges to normal operation."[3]Threats and challenges for services can range from simple misconfiguration over large scale natural disasters to targeted attacks.[4]As such, network resilience touches a very wide range of topics. In order to increase the resilience of a given communication network, the probable challenges and risks have to be identified and appropriate resilience metrics have to be defined for the service to be protected.[5] The importance of network resilience is continuously increasing, as communication networks are becoming a fundamental component in the operation of critical infrastructures.[6]Consequently, recent efforts focus on interpreting and improving network and computing resilience with applications to critical infrastructures.[7]As an example, one can consider as a resilience objective the provisioning of services over the network, instead of the services of the network itself. This may require coordinated response from both the network and from the services running on top of the network.[8] These services include: Resilience andsurvivabilityare interchangeably used according to the specific context of a given study.[9] There are three principles ofsystems designinreliability engineeringthat can help achieve high availability. A distinction can be made between scheduled and unscheduled downtime. Typically,scheduled downtimeis a result ofmaintenancethat is disruptive to system operation and usually cannot be avoided with a currently installed system design. Scheduled downtime events might include patches tosystem softwarethat require arebootor system configuration changes that only take effect upon a reboot. In general, scheduled downtime is usually the result of some logical, management-initiated event. Unscheduled downtime events typically arise from some physical event, such as a hardware or software failure or environmental anomaly. Examples of unscheduled downtime events include power outages, failedCPUorRAMcomponents (or possibly other failed hardware components), an over-temperature related shutdown, logically or physically severed network connections, security breaches, or variousapplication,middleware, andoperating systemfailures. If users can be warned away from scheduled downtimes, then the distinction is useful. But if the requirement is for true high availability, then downtime is downtime whether or not it is scheduled. Many computing sites exclude scheduled downtime from availability calculations, assuming that it has little or no impact upon the computing user community. By doing this, they can claim to have phenomenally high availability, which might give the illusion ofcontinuous availability. Systems that exhibit truly continuous availability are comparatively rare and higher priced, and most have carefully implemented specialty designs that eliminate anysingle point of failureand allow online hardware, network, operating system,middleware, and application upgrades, patches, and replacements. For certain systems, scheduled downtime does not matter, for example, system downtime at an office building after everybody has gone home for the night. Availability is usually expressed as a percentage of uptime in a given year. The following table shows the downtime that will be allowed for a particular percentage of availability, presuming that the system is required to operate continuously.Service level agreementsoften refer to monthly downtime or availability in order to calculate service credits to match monthly billing cycles. The following table shows the translation from a given availability percentage to the corresponding amount of time a system would be unavailable. The termsuptimeandavailabilityare often used interchangeably but do not always refer to the same thing. For example, a system can be "up" with its services not "available" in the case of anetwork outage. Or a system undergoing software maintenance can be "available" to be worked on by asystem administrator, but its services do not appear "up" to theend useror customer. The subject of the terms is thus important here: whether the focus of a discussion is the server hardware, server OS, functional service, software service/process, or similar, it is only if there is a single, consistent subject of the discussion that the words uptime and availability can be used synonymously. A simple mnemonic rule states that5 ninesallows approximately 5 minutes of downtime per year. Variants can be derived by multiplying or dividing by 10: 4 nines is 50 minutes and 3 nines is 500 minutes. In the opposite direction, 6 nines is 0.5 minutes (30 sec) and 7 nines is 3 seconds. Another memory trick to calculate the allowed downtime duration for an "n{\displaystyle n}-nines" availability percentage is to use the formula8.64×104−n{\displaystyle 8.64\times 10^{4-n}}seconds per day. For example, 90% ("one nine") yields the exponent4−1=3{\displaystyle 4-1=3}, and therefore the allowed downtime is8.64×103{\displaystyle 8.64\times 10^{3}}seconds per day. Also, 99.999% ("five nines") gives the exponent4−5=−1{\displaystyle 4-5=-1}, and therefore the allowed downtime is8.64×10−1{\displaystyle 8.64\times 10^{-1}}seconds per day. Percentages of a particular order of magnitude are sometimes referred to by thenumber of ninesor "class of nines" in the digits. For example, electricity that is delivered without interruptions (blackouts,brownoutsorsurges) 99.999% of the time would have 5 nines reliability, or class five.[10]In particular, the term is used in connection withmainframes[11][12]or enterprise computing, often as part of aservice-level agreement. Similarly, percentages ending in a 5 have conventional names, traditionally the number of nines, then "five", so 99.95% is "three nines five", abbreviated 3N5.[13][14]This is casually referred to as "three and a half nines",[15]but this is incorrect: a 5 is only a factor of 2, while a 9 is a factor of 10, so a 5 is 0.3 nines (per below formula:log10⁡2≈0.3{\displaystyle \log _{10}2\approx 0.3}):[note 2]99.95% availability is 3.3 nines, not 3.5 nines.[16]More simply, going from 99.9% availability to 99.95% availability is a factor of 2 (0.1% to 0.05% unavailability), but going from 99.95% to 99.99% availability is a factor of 5 (0.05% to 0.01% unavailability), over twice as much.[note 3] A formulation of theclass of 9sc{\displaystyle c}based on a system'sunavailabilityx{\displaystyle x}would be (cf.Floor and ceiling functions). Asimilar measurementis sometimes used to describe the purity of substances. In general, the number of nines is not often used by a network engineer when modeling and measuring availability because it is hard to apply in formula. More often, the unavailability expressed as aprobability(like 0.00001), or adowntimeper year is quoted. Availability specified as a number of nines is often seen inmarketingdocuments.[citation needed]The use of the "nines" has been called into question, since it does not appropriately reflect that the impact of unavailability varies with its time of occurrence.[17]For large amounts of 9s, the "unavailability" index (measure of downtime rather than uptime) is easier to handle. For example, this is why an "unavailability" rather than availability metric is used in hard disk or data linkbit error rates. Sometimes the humorous term "nine fives" (55.5555555%) is used to contrast with "five nines" (99.999%),[18][19][20]though this is not an actual goal, but rather a sarcastic reference to something totally failing to meet any reasonable target. Availability measurement is subject to some degree of interpretation. A system that has been up for 365 days in a non-leap year might have been eclipsed by a network failure that lasted for 9 hours during a peak usage period; the user community will see the system as unavailable, whereas the system administrator will claim 100% uptime. However, given the true definition of availability, the system will be approximately 99.9% available, or three nines (8751 hours of available time out of 8760 hours per non-leap year). Also, systems experiencing performance problems are often deemed partially or entirely unavailable by users, even when the systems are continuing to function. Similarly, unavailability of select application functions might go unnoticed by administrators yet be devastating to users – a true availability measure is holistic. Availability must be measured to be determined, ideally with comprehensive monitoring tools ("instrumentation") that are themselves highly available. If there is a lack of instrumentation, systems supporting high volume transaction processing throughout the day and night, such as credit card processing systems or telephone switches, are often inherently better monitored, at least by the users themselves, than systems which experience periodic lulls in demand. An alternative metric ismean time between failures(MTBF). Recovery time (or estimated time of repair (ETR), also known asrecovery time objective(RTO) is closely related to availability, that is the total time required for a planned outage or the time required to fully recover from an unplanned outage. Another metric ismean time to recovery(MTTR). Recovery time could be infinite with certain system designs and failures, i.e. full recovery is impossible. One such example is a fire or flood that destroys a data center and its systems when there is no secondarydisaster recoverydata center. Another related concept isdata availability, that is the degree to whichdatabasesand other information storage systems faithfully record and report system transactions. Information management often focuses separately on data availability, orRecovery Point Objective, in order to determine acceptable (or actual)data losswith various failure events. Some users can tolerate application service interruptions but cannot tolerate data loss. Aservice level agreement("SLA") formalizes an organization's availability objectives and requirements. High availability is one of the primary requirements of thecontrol systemsinunmanned vehiclesandautonomous maritime vessels. If the controlling system becomes unavailable, theGround Combat Vehicle(GCV) orASW Continuous Trail Unmanned Vessel(ACTUV) would be lost. On one hand, adding more components to an overall system design can undermine efforts to achieve high availability becausecomplex systemsinherently have more potential failure points and are more difficult to implement correctly. While some analysts would put forth the theory that the most highly available systems adhere to a simple architecture (a single, high-quality, multi-purpose physical system with comprehensive internal hardware redundancy), this architecture suffers from the requirement that the entire system must be brought down for patching and operating system upgrades. More advanced system designs allow for systems to be patched and upgraded without compromising service availability (seeload balancingandfailover). High availability requires less human intervention to restore operation in complex systems; the reason for this being that the most common cause for outages is human error.[21] On the other hand,redundancyis used to create systems with high levels of availability (e.g. popular ecommerce websites). In this case it is required to have high levels of failure detectability and avoidance of common cause failures. If redundant parts are usedin parallel and have independent failure(e.g. by not being within the same data center), they can exponentially increase the availability and make the overall system highly available. If you have N parallel components each having X availability, then you can use following formula:[22][23] Availability of parallel components = 1 - (1 - X)^ N So for example if each of your components has only 50% availability, by using 10 of components in parallel, you can achieve 99.9023% availability. Two kinds of redundancy are passive redundancy and active redundancy. Passive redundancy is used to achieve high availability by including enough excess capacity in the design to accommodate a performance decline. The simplest example is a boat with two separate engines driving two separate propellers. The boat continues toward its destination despite failure of a single engine or propeller. A more complex example is multiple redundant power generation facilities within a large system involvingelectric power transmission. Malfunction of single components is not considered to be a failure unless the resulting performance decline exceeds the specification limits for the entire system. Active redundancy is used in complex systems to achieve high availability with no performance decline. Multiple items of the same kind are incorporated into a design that includes a method to detect failure and automatically reconfigure the system to bypass failed items using a voting scheme. This is used with complex computing systems that are linked. Internetroutingis derived from early work by Birman and Joseph in this area.[24][non-primary source needed]Active redundancy may introduce more complex failure modes into a system, such as continuous system reconfiguration due to faulty voting logic. Zero downtime system design means that modeling and simulation indicates mean time between failures significantly exceeds the period of time betweenplanned maintenance,upgradeevents, or system lifetime. Zero downtime involves massive redundancy, which is needed for some types of aircraft and for most kinds ofcommunications satellites.Global Positioning Systemis an example of a zero downtime system. Faultinstrumentationcan be used in systems with limited redundancy to achieve high availability. Maintenance actions occur during brief periods of downtime only after a fault indicator activates. Failure is only significant if this occurs during amission criticalperiod. Modeling and simulationis used to evaluate the theoretical reliability for large systems. The outcome of this kind of model is used to evaluate different design options. A model of the entire system is created, and the model is stressed by removing components. Redundancy simulation involves the N-x criteria. N represents the total number of components in the system. x is the number of components used to stress the system. N-1 means the model is stressed by evaluating performance with all possible combinations where one component is faulted. N-2 means the model is stressed by evaluating performance with all possible combinations where two component are faulted simultaneously. A survey among academic availability experts in 2010 ranked reasons for unavailability of enterprise IT systems. All reasons refer tonot following best practicein each of the following areas (in order of importance):[25] A book on the factors themselves was published in 2003.[26] In a 1998 report fromIBM Global Services, unavailable systems were estimated to have cost American businesses $4.54 billion in 1996, due to lost productivity and revenues.[27]	https://en.wikipedia.org/wiki/Resilience_(network)
Collective intelligenceCollective actionSelf-organized criticalityHerd mentalityPhase transitionAgent-based modellingSynchronizationAnt colony optimizationParticle swarm optimizationSwarm behaviour Social network analysisSmall-world networksCentralityMotifsGraph theoryScalingRobustnessSystems biologyDynamic networks Evolutionary computationGenetic algorithmsGenetic programmingArtificial lifeMachine learningEvolutionary developmental biologyArtificial intelligenceEvolutionary robotics Reaction–diffusion systemsPartial differential equationsDissipative structuresPercolationCellular automataSpatial ecologySelf-replication Conversation theoryEntropyFeedbackGoal-orientedHomeostasisInformation theoryOperationalizationSecond-order cyberneticsSelf-referenceSystem dynamicsSystems scienceSystems thinkingSensemakingVariety Ordinary differential equationsPhase spaceAttractorsPopulation dynamicsChaosMultistabilityBifurcation Rational choice theoryBounded rationality Incomputer science,robustnessis the ability of a computer system to cope witherrorsduringexecution[1][2]and cope with erroneous input.[2]Robustness can encompass many areas of computer science, such asrobust programming,robust machine learning, andRobust Security Network. Formal techniques, such asfuzz testing, are essential to showing robustness since this type of testing involves invalid or unexpected inputs. Alternatively,fault injectioncan be used to test robustness. Various commercial products perform robustness testing of software analysis.[3] In general, building robust systems that encompass every point of possible failure is difficult because of the vast quantity of possible inputs and input combinations.[4]Since all inputs and input combinations would require too much time to test, developers cannot run through all cases exhaustively. Instead, the developer will try to generalize such cases.[5]For example, imagine inputting someinteger values. Some selected inputs might consist of a negative number, zero, and a positive number. When using these numbers to test software in this way, the developer generalizes the set of all reals into three numbers. This is a more efficient and manageable method, but more prone to failure. Generalizing test cases is an example of just one technique to deal with failure—specifically, failure due to invalid user input. Systems generally may also fail due to other reasons as well, such as disconnecting from a network. Regardless, complex systems should still handle any errors encountered gracefully. There are many examples of such successful systems. Some of the most robust systems are evolvable and can be easily adapted to new situations.[4] Programs and software are tools focused on a very specific task, and thus are not generalized and flexible.[4]However, observations in systems such as theinternetorbiological systemsdemonstrate adaptation to their environments. One of the ways biological systems adapt to environments is through the use ofredundancy.[4]Many organs are redundant in humans. Thekidneyis one such example.Humansgenerally only need one kidney, but having a second kidney allows room for failure. This same principle may be taken to apply to software, but there are some challenges. When applying the principle of redundancy to computer science, blindly adding code is not suggested. Blindly adding code introduces more errors, makes the system more complex, and renders it harder to understand.[6]Code that does not provide any reinforcement to the already existing code is unwanted. The new code must instead possess equivalentfunctionality, so that if a function is broken, another providing the same function can replace it, using manual or automatedsoftware diversity. To do so, the new code must know how and when to accommodate the failure point.[4]This means morelogicneeds to be added to the system. But as a system adds more logic,components, and increases in size, it becomes more complex. Thus, when making a more redundant system, the system also becomes more complex and developers must consider balancing redundancy with complexity. Currently, computer science practices do not focus on building robust systems.[4]Rather, they tend to focus onscalabilityandefficiency. One of the main reasons why there is no focus on robustness today is because it is hard to do in a general way.[4] Robust programming is a style of programming that focuses on handling unexpected termination and unexpected actions.[7]It requires code to handle these terminations and actions gracefully by displaying accurate and unambiguouserror messages. These error messages allow the user to more easily debug the program. Robust machine learning typically refers to the robustness of machine learning algorithms. For a machine learning algorithm to be considered robust, either the testing error has to be consistent with the training error, or the performance is stable after adding some noise to the dataset.[8]Recently, consistently with their rise in popularity, there has been an increasing interest in the robustness of neural networks. This is particularly due their vulnerability to adverserial attacks.[9] Robust network design is the study of network design in the face of variable or uncertain demands.[10]In a sense, robustness in network design is broad just like robustness in software design because of the vast possibilities of changes or inputs. There exist algorithms that tolerate errors in the input.[11]	https://en.wikipedia.org/wiki/Robustness_(computer_science)
Indatabasetechnologies, arollbackis an operation which returns the database to some previous state. Rollbacks are important for databaseintegrity, because they mean that the database can be restored to a clean copy even after erroneous operations are performed.[1]They are crucial for recovering from database server crashes; by rolling back anytransactionwhich was active at the time of the crash, the database is restored to a consistent state. The rollback feature is usually implemented with atransaction log, but can also be implemented viamultiversion concurrency control. A cascading rollback occurs in database systems when a transaction (T1) causes a failure and a rollback must be performed. Other transactions dependent on T1's actions must also be rollbacked due to T1's failure, thus causing a cascading effect. That is, one transaction's failure causes many to fail. Practical database recovery techniques guarantee cascadeless rollback, therefore a cascading rollback is not a desirable result. Cascading rollback is scheduled by dba. SQL refers to Structured Query Language, a kind of language used to access, update and manipulate database. InSQL,ROLLBACKis a command that causes all data changes since the lastSTART TRANSACTIONorBEGINto be discarded by therelational database management systems(RDBMS), so that the state of the data is "rolled back" to the way it was before those changes were made.[2] AROLLBACKstatement will also release any existingsavepointsthat may be in use. In most SQL dialects,ROLLBACKs are connection specific. This means that if two connections are made to the same database, aROLLBACKmade in one connection will not affect any other connections. This is vital for properconcurrency. Rollbacks are not exclusive to databases: anystatefuldistributed systemmay use rollback operations to maintainconsistency. Examples of distributed systems that can support rollbacks includemessage queuesandworkflow management systems. More generally, any operation that resets a system to its previous state before another operation or series of operations can be viewed as a rollback.	https://en.wikipedia.org/wiki/Rollback_(data_management)

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